https://leetcode.com/problems/path-sum-iii/

class Solution {
	            public int pathSum(TreeNode root, int sum) {
	                HashMap<Integer, Integer> preSum = new HashMap<>();
	                preSum.put(0,1);
	                return helper(root,sum,0,preSum);
	            }
	            private int helper(TreeNode root,int sum,int curSum,HashMap<Integer,Integer> preSum){
	                if(root == null) return 0;
	                curSum += root.val;
	                int res = preSum.getOrDefault(curSum - sum, 0);
	                preSum.put(curSum, preSum.getOrDefault(curSum, 0) + 1);
	                res += helper(root.left,sum,curSum,preSum) + helper(root.right,sum,curSum,preSum);
	                preSum.put(curSum, preSum.get(curSum) - 1);
	                return res;
	            }
	        }
    
    
    class Solution {
        public int pathSum(TreeNode root, int sum) {
            if(root == null) return 0;
            return pathSumFrom(root,sum) + pathSum(root.left,sum) + pathSum(root.right,sum);
        }
        public int pathSumFrom(TreeNode root, int sum){
            if(root == null) return 0;
            return (root.val == sum ? 1 : 0) 
                + pathSumFrom(root.left, sum - root.val) + pathSumFrom(root.right, sum - root.val);
        }
    }