LeetCode 437

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    int path = 0;
    public int pathSum(TreeNode root, int sum) {
        if(root == null){
            return 0;
        }
        countSum(root, sum);
        pathSum(root.left, sum);
        pathSum(root.right, sum);
        return path;
    }

    private void countSum(TreeNode root, int sum){
        if(root == null){
            return;
        }
        sum  -= root.val;
        if(sum == 0){
            path++;
        }
        countSum(root.left,sum);
        countSum(root.right,sum);
    }
}