Completed Hashing-2 problems

Subarray sum equals k.java

@@ -0,0 +1,28 @@
+// TimeComplexity: O(n)
+// SpaceComplexity: O(n)
+// Did this code successfully run on Leetcode : yes
+// Three sentences abou the solution: We count the number of subarrays whose sum equals k using a prefix-sum approach.It keeps a running sum and uses a HashMap to store how many times each prefix sum has occurred.
+// If (currentSum − k) exists in the map, it means a subarray ending at the current index sums to k.
+// The map is updated at each step with the current prefix sum count. Finally, we return the total number of such subarrays found.
+
+class Solution {
+    public int subarraySum(int[] nums, int k) {
+        int count =0;
+        int sum =0;
+        HashMap<Integer,Integer> map = new HashMap<>();
+        map.put(0,1);
+        for(int i=0; i<nums.length; i++){
+            sum = sum +nums[i];
+            if(map.containsKey(sum-k)) {
+                int val = map.get(sum-k);
+                count = count +val;
+            }
+            if(map.containsKey(sum)) {
+                map.put(sum, map.get(sum) +1);
+            } else {
+                map.put(sum,1);
+            }
+        }
+        return count;
+    }
+}

contiguous array.java

@@ -0,0 +1,24 @@
+// TimeComplexity: O(n)
+// SpaceComplexity: O(n)
+// Did this code successfully run on Leetcode : yes
+// Any problem you faced while coding this : no
+
+class Solution {
+    public int findMaxLength(int[] nums) {
+        int max = 0;
+        int sum = 0;
+        HashMap<Integer, Integer> map = new HashMap<>();
+        map.put(0,-1);
+        for(int i=0; i<nums.length; i++) {
+            sum = nums[i] == 0 ? sum-1 :sum+1;
+            if(map.containsKey(sum)) {
+                int curr = i - map.get(sum);
+                max = Math.max(max,curr);
+
+            } else {
+                map.put(sum,i);
+            }
+        }
+        return max;
+    }
+}

longest palindrome.java

@@ -0,0 +1,24 @@
+// TimeComplexity: O(n)
+// SpaceComplexity: O(n)
+// Did this code successfully run on Leetcode : yes
+// Any problem you faced while coding this : no
+
+class Solution {
+    public int longestPalindrome(String s) {
+        HashSet<Character> set = new HashSet<>();
+        int count =0;
+        for(int i=0; i<s.length(); i++) {
+            char ch =  s.charAt(i);
+            if(set.contains(ch)){
+                count =count+2;
+                set.remove(ch);
+            } else{
+                set.add(ch);
+            }
+        }
+        if(!set.isEmpty()){
+            count=count+1;
+        }
+        return count;
+    }
+}