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
 \
  5
All 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;
    }
};