Path Sum

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.

题目大意：给定二叉树，判断是否存在根-叶路径，使得该路径上的值的和等于目标值

题目难度：Easy

/**
 * Created by gzdaijie on 16/6/9
 */
public class Solution {
    public boolean hasPathSum(TreeNode root, int sum) {
        if (root == null) return false;
        if (root.left == null && root.right == null) return sum == root.val;
        if (root.left != null && hasPathSum(root.left, sum - root.val)) return true;
        return root.right != null && hasPathSum(root.right, sum - root.val);
    }
}