112. Path Sum

Given a binary tree and a sum, determine if the tree has a root-to-leaf path such that adding up all the values along the path equals the given sum.

For example:
Given the below binary tree and sum = 22, return true, as there exist a root-to-leaf path 5->4->11->2 which sum is 22.

 

//一路减下去直到减为0

class Solution {

    public boolean hasPathSum(TreeNode root, int sum) {

        if (root == null) return false;

        if (root.left == null && root.right == null && sum - root.val == 0) return true;

        return  hasPathSum(root.left, sum - root.val) || hasPathSum(root.right, sum - root.val);

    }

}