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.

              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.

思路：使用普通的深度优先搜索即可解决问题。

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
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;
        
	// 下述写法有点化蛇添足，会导致找到一条路径之后，再寻找其他路径
        // bool left =hasPathSum(root->left, sum - root->val);
        // bool right = hasPathSum(root->right, sum - root->val);
        // return (left || right);
	return hasPathSum(root->left, sum - root->val) || hasPathSum(root->right, sum - root->val);
    }
};