Done with Hashing-2

Problem1.java

@@ -0,0 +1,23 @@
+import java.util.HashMap;
+
+class Solution {
+    public int subarraySum(int[] nums, int k) {
+        int rsum=0;
+        int n= nums.length;
+        int count = 0;
+        HashMap <Integer,Integer> map=new HashMap<>();
+
+        map.put(0,1);
+
+        for(int i=0; i<n;i++){
+            rsum += nums[i];
+            //if map conatins y (for ex y = x-z) then increase the count as per frequency
+            if(map.containsKey(rsum - k)){
+                count+= map.get(rsum-k);
+            }
+            //put (rsum and frequency of rsum occcureance)
+                map.put(rsum, map.getOrDefault(rsum, 0)+1);
+        }
+        return count;
+    }
+}

Problem2.java

@@ -0,0 +1,30 @@
+import java.util.HashMap;
+
+class Solution {
+    public int findMaxLength(int[] nums) {
+        if(nums==null  || nums.length==0) return 0;
+        HashMap<Integer, Integer> map=new HashMap<>();
+        int max=0;
+        int rsum=0; //runnung sum
+        map.put(0,-1);
+        for(int i=0;i<nums.length;i++){
+            if(nums[i]==0){
+                //decrease running sum by 1 if 0 is enountered
+                rsum -=1 ;
+            }
+            else{
+                //increase running sum by 1 if 1 is enountered
+                rsum += 1;
+            }
+            if(map.containsKey(rsum)){
+                int curr=i-map.get(rsum);
+                max= Math.max(max,curr);
+            }
+            else{
+                map.put(rsum,i);
+            }
+        }
+    return max;
+
+    }
+}

Problem3.java

@@ -0,0 +1,27 @@
+import java.util.*;
+
+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 hashset contains the ch already then increase the count variable by 2
+            if(set.contains(ch)){
+                count+=2;
+                //after count going to even number, remove from set
+                set.remove(ch);
+            }
+            //if set does not have that character then add it
+            else{
+                set.add(ch);
+            }
+        //If set is not empty, increase the count variable by 1    
+        }if(!set.isEmpty()){
+            count++;
+        }
+        //at the end return the value of count variable
+        return count;
+    }
+}