【LeetCode】257. Binary Tree Paths

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"]

提示：

此题如果另外再创建一个用于递归的函数来做的话，会比较简单，下面给出利用了递归的代码。

代码：

/**
 * 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:
    void recursiveTree(vector<string> &res, TreeNode* node, string str) {
        if (node->left == NULL && node->right == NULL) {
            res.push_back(str);
            return;
        }
        if (node->left != NULL) {
            recursiveTree(res, node->left, str + "->" + to_string(node->left->val));
        }
        if (node->right != NULL) {
            recursiveTree(res, node->right, str + "->" + to_string(node->right->val));
        }
    }
    
    vector<string> binaryTreePaths(TreeNode* root) {
        vector<string> res;
        if (root == NULL) return res;
        recursiveTree(res, root, to_string(root->val));
        return res;
    }
};