LeetCode 112. Path Sum

题目：

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.

思路：用sum减去当前节点的值，直到遇到一个叶子节点，并且此时差值为0。在做题的时候，我犯了一个错误，忽略了叶子节点的定义。

叶子节点是度数为0的节点。

代码AC：

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public boolean hasPathSum(TreeNode root, int sum) {                    if(root == null)   return false;   else if(root.val == sum && root.left==null && root.right == null)   return true;      else   return (hasPathSum(root.left, sum-root.val) ||   hasPathSum(root.right, sum-root.val));    }}