leetcode 257. Binary Tree Paths

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"]

题目大意：
输出二叉树的所有路径
思路：
先序遍历，记录路径。为了回溯，临时存储路径的数据结构得具有弹出最后一个元素的功能,我的代码里面用的是vector

/**
 * 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> v;
    void dfs(TreeNode* root, vector<string> s) {
        if (root == nullptr) return;
        if ((root->left == nullptr) && (root->right == nullptr)) {
            if(s.size() == 0) s.push_back(to_string(root->val));
            else s.push_back("->" + to_string(root->val));
            string t = "";
            for (int i = 0; i < s.size(); ++i) {
                t += s[i];
            }
            v.push_back(t);
            return ;
        }
        if(s.size() == 0) s.push_back(to_string(root->val));
        else s.push_back("->" + to_string(root->val));
        dfs(root->left, s);
        dfs(root->right, s);
        s.pop_back();
    }
    vector<string> binaryTreePaths(TreeNode* root) {
        vector<string> s;
        dfs(root, s);
        return v;
    }
};