[leetcode-112]Path Sum(java)

问题描述：
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,
5
/ \
4 8
/ / \
11 13 4
/ \ \
7 2 1
return true, as there exist a root-to-leaf path 5->4->11->2 which sum is 22.

分析：这道题要注意，路径中可能存在正数和负数，所以不能剪枝。此外，递归到什么程度，应该递归到叶节点。当然如果遇到null时，直接返回false即可。为啥要递归到叶节点而不是null呢？因为递归到null不能说明进行到了叶节点，此时不能拿sum 和 now的大小说事。

代码如下：364ms

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    private boolean checkPathSum(TreeNode root,int sum,int now){        if(root==null)            return false;        now += root.val;        //leaf node        if(root.left==null && root.right==null){            return sum==now;        }        return checkPathSum(root.left,sum,now) || checkPathSum(root.right,sum,now);    }    public boolean hasPathSum(TreeNode root, int sum) {        if(root==null)            return false;        return checkPathSum(root,sum,0);    }}