leetcode 257. Binary Tree Paths

解题思路：
深度优先，递归遍历二叉树，用path保存当前已经访问过的节点，如果当前节点是叶子节点，则将当前已经访问过的节点作为符合要求的路径加入结果集中。如果当前节点不是叶子节点，递归遍历其左子树，然后递归遍历其右子树。
注意一下代码中，形参path不要用引用
原题目：

Given a binary tree, return all root-to-leaf paths.For example, given the following binary tree:   1 /   \2     3 \  5All root-to-leaf paths are:["1->2->5", "1->3"]

AC解，C++代码，菜鸟一个，请大家多多指正

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    vector<string> ret;    stringstream stream;    void binaryTreePaths(TreeNode* root, string path) {        if (root == NULL) {            return;                }        else {            if (!path.empty()) {                path += "->";            }            string tmp;            stream.clear();            stream << root->val;            stream >> tmp;            path += tmp;            if(root->left == NULL && root->right == NULL) {                ret.push_back(path);              }            else {                binaryTreePaths(root->left, path);                binaryTreePaths(root->right, path);                           }        }    }    vector<string> binaryTreePaths(TreeNode* root) {       string path;       binaryTreePaths(root, path);       return ret;    }};