diff --git a/Leetcode_409.java b/Leetcode_409.java
new file mode 100644
index 00000000..4b541604
--- /dev/null
+++ b/Leetcode_409.java
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+//Rule: Lookslike a mirror immage. i.e., replica. So, we should have same characters at both front side and back side. So, there should be one char with odd times and rest all should be even times.
+//Solving it using set
+//TC: O(n); SC:O(n)
+class Solution {
+    public int longestPalindrome(String s) {
+        Set<Character> set=new HashSet<>();
+
+        int ans=0;
+
+        for(int i=0;i<s.length();i++){
+            char ch=s.charAt(i);
+            if(set.contains(ch)){
+                set.remove(ch);
+                ans+=2;
+            }else{
+                set.add(ch);
+            }
+
+        }
+
+        if(set.size()>0) ans+=1;
+
+        return ans;
+        
+    }
+}
diff --git a/Leetcode_525.java b/Leetcode_525.java
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index 00000000..65f7ec82
--- /dev/null
+++ b/Leetcode_525.java
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+//Rule: Contiguous array-->Number of 1's is equals to number of 0's. 
+//Let's consider 1's as increments and 0 as decrements. As number of 1's = number of 0's, number of increments is equals to number of decrements.
+//The indexes at which prefix sum is equals will have equal number of 0's and 1's between them.
+//prefix sum is calculated using sum variable. And traking the indexes with similar sum using hashmap
+//TC: O(n); SC: O(n)
+class Solution {
+    public int findMaxLength(int[] nums) {
+        Map<Integer,Integer> map=new HashMap<>();
+        map.put(0,-1);
+        int sum=0;
+        int ans=0;
+
+        for(int i=0;i<nums.length;i++){
+            if(nums[i]==0){
+                sum=sum-1;
+            }else{
+                sum=sum+1;
+            }
+
+            if(map.containsKey(sum)){
+                ans=Math.max(ans,(i-map.get(sum)));
+            }else{
+                map.put(sum,i);
+            }
+        }
+
+        return ans;
+        
+    }
+}
diff --git a/Leetcode_560.java b/Leetcode_560.java
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index 00000000..2615397d
--- /dev/null
+++ b/Leetcode_560.java
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+//As they are asking subarray sum, have a variable and maintain sum in it at every index. At every index, we need some amount to match with target. That amount can be calculated as sum-target. 
+//To check whether we have any sum-target in the array, we will maintain a map and count number of occurances of quantity(sum) at every index. If it's present, that many times we have the possibility to reach target.
+//Tc: O(n)
+//Sc: O(n)
+
+class Solution {
+    public int subarraySum(int[] nums, int k) {
+        Map<Integer,Integer> map=new HashMap<>();
+        map.put(0,1);
+        int count=0;
+        int sum=0;
+
+        for(int i:nums){
+            sum+=i;
+            int target=sum-k;
+            if(map.containsKey(target)){
+                count+=map.get(target);
+            }
+
+            map.put(sum,map.getOrDefault(sum,0)+1);
+        }
+
+        return count;
+        
+    }
+}