├── Day01_SortThePeople.java
├── Day02_SortArrayByIncreasingFrequency.java
├── Day03_SortTheJumbledNumbers.java
├── Day04_SortAnArray.java
├── Day05_FindTheCitySmallestNoOFNeibours.java
├── Day06_MinimumCostToConvertString.java
├── Day07_ReverseString.java
├── Day08_FindTheDifference.java
├── Day09_SingleNumber.java
├── Day10_SoreOfAString.java
├── Day11_SmallestEvenMultiple
├── Day12_SumMultiples.java
├── Day13_PowerOfTwo.java
├── Day14_RotateString.java
├── Day15_PlusOne.java
├── Day16_ShuffleString.java
├── Day17_ClearDigits.java
├── Day18_LemonadeChange.java
├── Day19_Convert1DArrayInto2DArray.java
├── Day20_FindTheStudentThatWillReplaceTheChalk.java
├── Day21_SumOfDigitsOfStringAfterCovert.java
├── Day22_NumberOf1Bits.java
├── Day23_Pow(x,n).java
├── Day24_ReverseInteger.java
├── Day25_ReverseBits.java
├── Day26_AddDigits.java
├── Day27_MissingNumber.java
├── Day28_ThirdMaximumNumber.java
├── Day29_PowerOfFour.java
├── Day30_HappyNumber.java
└── README.md


/Day01_SortThePeople.java:
--------------------------------------------------------------------------------
 1 | class Solution {
 2 |     public String[] sortPeople(String[] names, int[] heights) {
 3 |    HashMap<Integer,String> map = new HashMap<Integer,String>();
 4 |        for(int i = 0;i<names.length;i++){
 5 |         map.put(heights[i],names[i]);
 6 |        }
 7 |        Arrays.sort(heights);
 8 |        String res[] = new String[heights.length];
 9 |        int j =0;
10 |        for(int i = heights.length-1;i>=0;i--){
11 |         res[j] = map.get(heights[i]);
12 |         j++;
13 |        }
14 |        return res;
15 |     }
16 | }
17 | 


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/Day02_SortArrayByIncreasingFrequency.java:
--------------------------------------------------------------------------------
 1 | class Solution {
 2 |     public int[] frequencySort(int[] nums) {
 3 |         Map<Integer, Integer> f =new HashMap<>();
 4 |         Integer [] newNums = new Integer[nums.length];
 5 | 
 6 |         for(int i=0; i<nums.length;i++){
 7 |             f.put(nums[i], f.getOrDefault(nums[i],0)+1);
 8 |             newNums[i]=nums[i];
 9 |         }
10 |         
11 |         Arrays.sort(newNums, (n1,n2) ->{
12 |             if (f.get(n1) != f.get(n2)){
13 |                 return f.get(n1)-f.get(n2);
14 |             }
15 |             else{
16 |                 return n2-n1;
17 |             }
18 |         });
19 |         for (int i = 0; i < nums.length; i++) {
20 |             nums[i] = newNums[i];
21 |         }
22 | 
23 |         return nums;
24 |         }
25 |     }
26 | 


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/Day03_SortTheJumbledNumbers.java:
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 1 | class Solution {
 2 |     public int[] sortJumbled(int[] mapping, int[] nums) {
 3 |         int pairs[] = new int[nums.length];
 4 |         HashMap<Integer,Integer> map = new HashMap<>(); 
 5 |          for(int i=0;i<nums.length;i++){
 6 |             int n=nums[i];
 7 |             int mapped_n=0;
 8 |             int base =1;
 9 |             if(n==0)
10 |             {
11 |                 mapped_n=mapping[n];
12 |             }
13 |             while(n>0){
14 |                 int digit=n%10;
15 |                 n=n/10;
16 |                 mapped_n+=base*mapping[digit];
17 |                 base*=10;
18 |             }
19 |             map.put(i,mapped_n);
20 |          }
21 |          Map<Integer,Integer> nmap = sortByValue(map);
22 |          for(int i=0;i<nums.length;i++)
23 |          {
24 |             pairs[i]=nums[nmap[i]];
25 |          }
26 |         return pairs;
27 |     }
28 | }
29 | 


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/Day04_SortAnArray.java:
--------------------------------------------------------------------------------
1 | class Solution:
2 |     def sortArray(self, nums: List[int]) -> List[int]:
3 |         return sorted(nums)
4 |         
5 | 


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/Day05_FindTheCitySmallestNoOFNeibours.java:
--------------------------------------------------------------------------------
 1 | class Solution {
 2 |     public int findTheCity(int n, int[][] edges, int distanceThreshold) {
 3 |         int[][] distance = new int[n][n];
 4 |         for (int i = 0; i < n; i++) {
 5 |             Arrays.fill(distance[i], Integer.MAX_VALUE);
 6 |             distance[i][i] = 0;  // distance to itself is 0
 7 |         }
 8 | 
 9 |         // The distance between nodes which are connected temporary distance between them
10 |         for (int[] edge : edges) {
11 |             int node1 = edge[0], node2 = edge[1], dist = edge[2];
12 |             distance[node1][node2] = dist;
13 |             distance[node2][node1] = dist;
14 |         }
15 | 
16 |         for (int midle = 0; midle < n; midle++) {
17 |             for (int source = 0; source < n; source++) {
18 |                 for (int destination = 0; destination < n; destination++) {
19 |                     if (distance[source][midle] < Integer.MAX_VALUE && distance[midle][destination] < Integer.MAX_VALUE) {
20 |                         distance[source][destination] = Math.min(
21 |                             distance[source][destination], distance[source][midle] + distance[midle][destination]
22 |                         );  // the minimum distance is either current value or new value with path that goes through midle
23 |                     }
24 |                 }
25 |             }
26 |         }
27 | 
28 |         int minimum_number = n;
29 |         int res = -1;
30 | 
31 |         for (int source = 0; source < n; source++) {
32 |             int source_count = 0;
33 |             for (int destination = 0; destination < n; destination++) {
34 |                 if (distance[source][destination] <= distanceThreshold) {
35 |                     source_count++;
36 |                 }
37 |             }
38 | 
39 |             if (source_count <= minimum_number) {  // as in dijkstra when number equal we choose greater node
40 |                 minimum_number = source_count;
41 |                 res = source;
42 |             }
43 |         }
44 | 
45 |         return res;
46 |     }
47 | }
48 | 


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/Day06_MinimumCostToConvertString.java:
--------------------------------------------------------------------------------
 1 | class Solution {
 2 |     public long minimumCost(String source, String target, char[] original, char[] changed, int[] cost) {
 3 |         int[][] dis = new int[26][26];
 4 |         for (int i = 0; i < 26; i++) {
 5 |             Arrays.fill(dis[i], Integer.MAX_VALUE);
 6 |             dis[i][i] = 0;
 7 |         }
 8 |         for (int i = 0; i < cost.length; i++) {
 9 |             dis[original[i] - 'a'][changed[i] - 'a'] = Math.min(dis[original[i] - 'a'][changed[i] - 'a'], cost[i]);
10 |         }
11 |         for (int k = 0; k < 26; k++) {
12 |             for (int i = 0; i < 26; i++)
13 |                 if (dis[i][k] < Integer.MAX_VALUE) {
14 |                     for (int j = 0; j < 26; j++) {
15 |                         if (dis[k][j] < Integer.MAX_VALUE) {
16 |                             dis[i][j] = Math.min(dis[i][j], dis[i][k] + dis[k][j]);
17 |                         }
18 |                     }
19 |                 }
20 |         }
21 |         long ans = 0L;
22 |         for (int i = 0; i < source.length(); i++) {
23 |             int c1 = source.charAt(i) - 'a';
24 |             int c2 = target.charAt(i) - 'a';
25 |             if (dis[c1][c2] == Integer.MAX_VALUE) {
26 |                 return -1L;
27 |             } else {
28 |                 ans += (long)dis[c1][c2];
29 |             }
30 |         }
31 |         return ans;
32 |     }
33 | }
34 | 


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/Day07_ReverseString.java:
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 1 | class Solution {
 2 |     public void reverseString(char[] s) {
 3 |         int l=s.length-1,i;
 4 |         for(i=0;i<s.length/2;i++)
 5 |         {
 6 |             char temp=s[i];
 7 |             s[i]=s[l];
 8 |             s[l]=temp;
 9 |             l--;
10 |         }
11 |     }
12 | }
13 | 


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/Day08_FindTheDifference.java:
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 1 | class Solution {
 2 |     public char findTheDifference(String s, String t) {
 3 |         int i=0,s1=0;
 4 |         for(i=0;i<s.length();i++)
 5 |         {
 6 |              s1+=(int)t.charAt(i);
 7 |              s1-=(int)s.charAt(i);
 8 |         }
 9 |         s1+=(int)t.charAt(i);
10 |         return (char)s1;
11 |     }
12 | }
13 | 


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/Day09_SingleNumber.java:
--------------------------------------------------------------------------------
 1 | class Solution {
 2 |     public int singleNumber(int[] nums) {
 3 |         int x=0;
 4 |         for(int i=0;i<nums.length;i++)
 5 |         {
 6 |             x=x^nums[i];
 7 |         }
 8 |         return x;    
 9 |     }
10 | }
11 | 


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/Day10_SoreOfAString.java:
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 1 | class Solution {
 2 |     public int scoreOfString(String s) {
 3 |         int sum=0,a,b;
 4 |         for(int i=1;i<s.length();i++)
 5 |         {
 6 |              a=(int)s.charAt(i-1);
 7 |              b=(int)s.charAt(i);
 8 |             sum=sum+(Math.abs(a-b));
 9 |         }
10 |         return sum;
11 |         
12 |     }
13 | }
14 | 


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/Day11_SmallestEvenMultiple:
--------------------------------------------------------------------------------
1 | class Solution {
2 |     public int smallestEvenMultiple(int n) {
3 |         if(n%2==0) return n;
4 |         else return n*2;
5 |         
6 |     }
7 | }
8 | 


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/Day12_SumMultiples.java:
--------------------------------------------------------------------------------
 1 | class Solution {
 2 |     public int sumOfMultiples(int n) {
 3 |         int s=0;
 4 |         for(int i=3;i<=n;i++)
 5 |         {
 6 |             if(i%3==0 || i%5==0 || i%7==0){
 7 |                 s=s+i;
 8 |             }
 9 |         }
10 |         return s; 
11 |     }
12 | }
13 | 


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/Day13_PowerOfTwo.java:
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 1 | // space complexity : O(n)
 2 | // time complexity  : O(logN)
 3 | class Solution {
 4 |     public boolean isPowerOfTwo(int n) {
 5 |         int i=1,j=0;
 6 |         while(i<=n && j<=31)
 7 |         {
 8 |             if(i==n) return(true);
 9 |             i=i*2;
10 |             j++;
11 |         }
12 |         return(false);
13 |     }
14 | }
15 | 
16 | // space complexity : O(n)
17 | // time complexity  : O(logN)
18 | class Solution {
19 |     public boolean isPowerOfTwo(int n) {
20 |      if(n<=0) return(false);
21 |      while(n%2==0)
22 |      {
23 |         n/=2;
24 |      }
25 |      return(n==1);
26 |     }
27 | }
28 | 
29 | 


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/Day14_RotateString.java:
--------------------------------------------------------------------------------
1 | class Solution {
2 |     public boolean rotateString(String s, String goal) {
3 | 
4 |        if(s.length()!=goal.length()) return false;
5 |        String str=s+s;
6 |        return str.contains(goal); 
7 |     }
8 | }
9 | 


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/Day15_PlusOne.java:
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 1 | class Solution {
 2 |     public int[] plusOne(int[] digits) {
 3 |         int n=digits.length-1;
 4 |         digits[n]+=1;
 5 |         int m=digits[n];
 6 |         while(n>=0 && m==10){
 7 |             if(n==0){
 8 |                 digits = new int[digits.length+1];
 9 |                 digits[n]=1;
10 |                 return(digits);
11 |             } 
12 |             else{
13 |                 digits[n-1]+=1;
14 |             }
15 |             digits[n]=0;
16 |             n--;
17 |             m=digits[n];
18 |         }
19 |         return(digits);
20 |         
21 |     }
22 | }
23 | 


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/Day16_ShuffleString.java:
--------------------------------------------------------------------------------
 1 | class Solution {
 2 |     public String restoreString(String s, int[] indices) {
 3 |         StringBuffer stri = new StringBuffer(s);
 4 |         for(int i=0;i<s.length();i++)
 5 |         {
 6 |             stri.delete(indices[i],indices[i]+1);
 7 |             stri.insert(indices[i],s.charAt(i));
 8 |         } 
 9 |         return stri.toString();
10 |     }
11 | }
12 | 


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/Day17_ClearDigits.java:
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 1 | class Solution {
 2 |     public String clearDigits(String s) {
 3 |          StringBuilder sb = new StringBuilder();
 4 |         for(int i=0;i<s.length();i++){
 5 |             if(s.charAt(i)>='0'&& s.charAt(i)<='9'){
 6 |                 sb.deleteCharAt(sb.length()-1);
 7 |               
 8 |             }
 9 |             else{
10 |                 sb.append(s.charAt(i));
11 |             }
12 |         }
13 |         return sb.toString();
14 |     }
15 | }
16 | 


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/Day18_LemonadeChange.java:
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 1 | class Solution {
 2 |     public boolean lemonadeChange(int[] bills) {
 3 |         int five=0;
 4 |         int ten =0;
 5 |         for(int i=0;i<bills.length;i++)
 6 |         {
 7 |             if(bills[i]==5) five+=5;
 8 | 
 9 |             else if(bills[i]==20 )
10 |             {
11 |                 if(ten>0) ten-=10;
12 |                 else five-=10;
13 |                 five-=5;      
14 |             }
15 |             else{
16 |                 ten+=10;
17 |                 five-=5;
18 |             }
19 |             if(five<0 || ten<0) return(false);
20 |         }
21 |         return(five>=0 && ten>=0);
22 |     }
23 | }
24 | 


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/Day19_Convert1DArrayInto2DArray.java:
--------------------------------------------------------------------------------
 1 | class Solution {
 2 |     public int[][] construct2DArray(int[] original, int m, int n) {
 3 |         int a[][] = new int[m][n];
 4 |         int k=0;
 5 |         if(m*n!=original.length) return new int[0][0];
 6 |         
 7 |         for(int i=0;i<m;i++){
 8 |             for(int j=0;j<n;j++){
 9 |                 a[i][j]=original[k];
10 |                 k++;  
11 |             }
12 |         }
13 |         return a;  
14 |     }
15 | }
16 | 


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/Day20_FindTheStudentThatWillReplaceTheChalk.java:
--------------------------------------------------------------------------------
 1 | class Solution {
 2 |     public int chalkReplacer(int[] chalk, int k) {
 3 |       
 4 |       // worst case
 5 |       
 6 |       // int i=0;
 7 |         // while(i<chalk.length)
 8 |         // {
 9 |         //     k-=chalk[i];
10 |         //     if(k<0) return i;
11 |         //     if(i==chalk.length-1){
12 |         //         i=0;
13 |         //         continue;
14 |         //     }
15 |         //     i++;
16 |         // }
17 |         // return i;
18 | 
19 |       // best case
20 |       
21 |       long s=0;  //Arrays.stream(chalk).sum();
22 |         for (int i : chalk) {
23 |             s += i;
24 |         }
25 |         k=(int)(k%s);
26 |         for(int i=0;i<chalk.length;i++)
27 |         {
28 |             if(k<chalk[i]) return i;
29 |             k-=chalk[i];
30 |         }
31 |         return 0;
32 |     }
33 | }
34 | 


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/Day21_SumOfDigitsOfStringAfterCovert.java:
--------------------------------------------------------------------------------
 1 | class Solution {
 2 |     public int getLucky(String s, int k) {
 3 |         
 4 |         StringBuilder number = new StringBuilder();
 5 |         for (char x : s.toCharArray()) {
 6 |             number.append(x-96);
 7 |         }
 8 | 
 9 |         while (k > 0) {
10 |             int temp = 0;
11 |             for (char x : number.toString().toCharArray()) {
12 |                 temp += x - '0';  
13 |             }
14 |             number = new StringBuilder(String.valueOf(temp)); 
15 |             k--;
16 |         }
17 |         return Integer.parseInt(number.toString()); 
18 |     }
19 | }
20 | 


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/Day22_NumberOf1Bits.java:
--------------------------------------------------------------------------------
 1 | class Solution {
 2 |     public int hammingWeight(int n) {
 3 |         int a=0;
 4 |         while(n>0)
 5 |         {
 6 |             if(n%2==1) a+=1;
 7 |             n/=2;
 8 |         }
 9 |         return a;
10 |     }
11 | }
12 | 


--------------------------------------------------------------------------------
/Day23_Pow(x,n).java:
--------------------------------------------------------------------------------
 1 | class Solution {
 2 |     public double myPow(double x, int n) {
 3 |     
 4 |         double ans = 1.0;
 5 |         long nTemp = n;
 6 | 
 7 |         if(n < 0)
 8 |             nTemp = nTemp * -1;
 9 | 
10 |         while(nTemp > 0){
11 |             if(nTemp % 2 == 0){
12 |                 x *= x;
13 |                 nTemp /=2;
14 |             }
15 |             else{
16 |                 ans *= x;
17 |                 nTemp--;
18 |             }
19 |         }
20 |         return n < 0 ? 1 / ans : ans;
21 |     }
22 | }
23 | 


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/Day24_ReverseInteger.java:
--------------------------------------------------------------------------------
 1 | class Solution {
 2 |     public int reverse(int x) {
 3 |         long sum=0; 
 4 |         while(x!=0)
 5 |         {
 6 |             sum= sum*10 + (x%10);
 7 |             x=x/10;
 8 |         }
 9 |         if(sum > Integer.MAX_VALUE || sum<Integer.MIN_VALUE){
10 |             return 0;
11 |         }
12 |         return (int)sum;
13 |     }
14 | }
15 | 


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/Day25_ReverseBits.java:
--------------------------------------------------------------------------------
 1 | 
 2 | // space
 3 | public class Solution {
 4 |     // you need treat n as an unsigned value
 5 |     public int reverseBits(int n) {
 6 |         int result =0;
 7 |         for(int i=0;i<32;i++)
 8 |         {
 9 |             result <<= 1;
10 |             result |= (n&1);
11 |             n >>= 1;
12 |         }
13 |         return result;
14 |        
15 |     }
16 | }
17 | 
18 | // time
19 | public class Solution {
20 |     // you need treat n as an unsigned value
21 |     public int reverseBits(int n) {
22 |         return Integer.reverse(n);
23 |     }
24 | }
25 | 


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/Day26_AddDigits.java:
--------------------------------------------------------------------------------
 1 | class Solution {
 2 |     public int addDigits(int num) {
 3 |         
 4 |         while(num/10 != 0)
 5 |         {
 6 |             int sum=0;
 7 |             while(num!=0)
 8 |             {
 9 |                 sum+=num%10;
10 |                 num/=10;
11 |             }
12 |             num=sum;
13 |         }
14 |         return num;
15 |         
16 |     }
17 | }
18 | 


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/Day27_MissingNumber.java:
--------------------------------------------------------------------------------
 1 | class Solution {
 2 |     public int missingNumber(int[] nums) {
 3 |         int i=0,j;
 4 |         int sum=0;
 5 |         for(j=0;j<nums.length;j++)
 6 |         {
 7 |             i+=j;
 8 |             sum+=nums[j];
 9 |         }
10 |         i+=j;
11 |         return i-sum;
12 |         
13 |     }
14 | }
15 | 


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/Day28_ThirdMaximumNumber.java:
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 1 | 
 2 | class Solution {
 3 |     public int thirdMax(int[] nums) {
 4 |         long max1=Long.MIN_VALUE;
 5 |         long max2=Long.MIN_VALUE;
 6 |         long max3=Long.MIN_VALUE;
 7 |         for(int num : nums){
 8 |             if(num>max1){
 9 |                 max3 = max2;
10 |                 max2 = max1;
11 |                 max1 = num;
12 |             }else if(max1>num && num>max2){
13 |                 max3 = max2;
14 |                 max2 = num;
15 |             }else if(max2>num && num>max3){
16 |                 max3=num;
17 |             }
18 |         }
19 |         return max3 != Long.MIN_VALUE ? (int) max3 : (int) max1;
20 |     }
21 | }
22 | 


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/Day29_PowerOfFour.java:
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 1 | class Solution {
 2 |     public boolean isPowerOfFour(int n) {
 3 |         if(n<=0) return false;
 4 |         while(n>1)
 5 |         {
 6 |             if(n%4!=0) return false;
 7 |             n=n/4;
 8 |         }
 9 |         return true;
10 |         
11 |     }
12 | }
13 | 


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/Day30_HappyNumber.java:
--------------------------------------------------------------------------------
 1 | class Solution {
 2 |     public boolean isHappy(int n) {
 3 |         int sum = 0;
 4 |         if (n == 1) {
 5 |             return true;
 6 |         } else if (n==4) {
 7 |             return false;
 8 |         } else {
 9 |             while (n > 0) {
10 |                 sum = sum + (int) Math.pow(n % 10, 2);
11 |                 n = n / 10;
12 |             }
13 |             return isHappy(sum);
14 |         }
15 |     }
16 | }
17 | 


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/README.md:
--------------------------------------------------------------------------------
 1 | # LeetCode 30-Day Challenge Solutions 🚀
 2 | 
 3 | ## 📌 Repository Overview
 4 | This repository is dedicated to solving **LeetCode Problem 30** as part of a **30-day coding challenge**. The goal of this challenge is to enhance problem-solving skills by consistently working through algorithmic problems over an extended period. Each day presents a new challenge that is carefully analyzed and solved with an optimized approach.
 5 | 
 6 | ## 📂 Contents
 7 | - **Problem Statement:** A brief description of the problem.
 8 | - **Approach & Solution:** Step-by-step explanation of the implemented solution.
 9 | - **Code Implementations:** Solutions in various programming languages (if applicable).
10 | - **Time & Space Complexity Analysis:** Optimization details.
11 | 
12 | ## 🚀 Technologies Used
13 | - **Programming Language:** Java
14 | - **Problems:** Algorithms & Data Structures
15 | 
16 | ## 🔥 How to Use
17 | 
18 | ### 1️⃣ Clone the repository
19 | ```sh
20 | git clone https://github.com/sriram2195/leetcode-java.git
21 | ```
22 | ### 2️⃣ Navigate to the project directory
23 | ```sh
24 | cd leetcode-java
25 | ```
26 | ### 3️⃣ Run the solution file
27 | ```sh
28 | python Day1_SortThePeople.java  # Example for Python
29 | ```
30 | 
31 | ## 📞 Contact
32 | For any queries or discussions, feel free to **open an issue** or **reach out**.
33 | 
34 | ---
35 | Happy Coding! 🚀
36 | 


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