原题链接在这里：https://leetcode.com/problems/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.
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.
题解：
减掉当前节点值，判断是不是叶子节点同时满足sum==0即可。
Time Complexity: O(n), worst case是每个node都跑了一次.
Space: O(logn), stack space.
AC Java:
1 /** 2 * Definition for a binary tree node. 3 * public class TreeNode { 4 * int val; 5 * TreeNode left; 6 * TreeNode right; 7 * TreeNode(int x) { val = x; } 8 * } 9 */ 10 public class Solution { 11 public boolean hasPathSum(TreeNode root, int sum) { 12 if(root == null){ 13 return false; 14 } 15 sum-=root.val; 16 if(root.left == null && root.right == null && sum == 0){ 17 return true; 18 }else{ 19 return hasPathSum(root.left,sum) || hasPathSum(root.right,sum); 20 } 21 } 22 }