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"]       

答案：
    和遍历类似，用递归的思路，深度优先搜索的算法

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