Path Sum

题目原型：

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.

基本思路：

题目基本意思就是说给定一个数sum，自顶向下求一条路径，使得这条路径的和为这个给定值。采用递归，没一层递归时，将sum-1，只要有一条路径符合即可。

public boolean hasPathSum(TreeNode root, int sum){if(root==null)return false;else if(root.left==null&&root.right==null){if(sum==root.val)return true;elsereturn false;}elsereturn hasPathSum(root.left, sum-root.val)||hasPathSum(root.right, sum-root.val);}