题：https://leetcode.com/problems/path-sum/description/

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.

Note: A leaf is a node with no children.

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.

题目大意

若存在 从 root 到某个 leaf 结点的和为 sum，返回 true，否则返回 false。

思路

递归，
递归函数 该root结点到 其 叶子结点 的和 是否有等于 sum。

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