[LeetCode116]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.

中文（判断二叉树一条路径和是否等于给定的值）

代码：

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