Prefix Sum

Subarray Sum Equals K

Given an array of integers nums and an integer k, return the total number of continuous subarrays whose sum equals to k.

class Solution {
    public int subarraySum(int[] nums, int k) {
        int count = 0;
        Map<Integer, Integer> map = new HashMap<>();
        map.put(0,1);
        int sum = 0;
        for(int i = 0; i < nums.length; i++) {
            sum+=nums[i];
            if(map.containsKey(sum-k)) {
                count+=map.get(sum-k);
            }
            map.put(sum, map.getOrDefault(sum, 0) + 1);
        }
        return count;
    }
}

Path sum 3

Given the root of a binary tree and an integer targetSum, return the number of paths where the sum of the values along the path equals targetSum.

The path does not need to start or end at the root or a leaf, but it must go downwards (i.e., traveling only from parent nodes to child nodes).

class Solution {
    HashMap<Integer,Integer> map;
    int target=0;
    int count=0;
    public int pathSum(TreeNode root, int targetSum) {
        map= new HashMap();
        map.put(0,1);
        this.target=targetSum;
        sum(root,0);
        return count;
        
    }
    public void sum(TreeNode root, int curr){
        if(root!=null){
            curr=curr+root.val;
            if(map.containsKey(curr-target)){
                count+=map.get(curr-target);
            }
            map.put(curr,map.getOrDefault(curr,0)+1);
            sum(root.left,curr);
            sum(root.right, curr);
            map.put(curr, map.get(curr)-1);
            
        }
    }
}