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"]

  /**
 * 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> res;
        if(root != NULL)
            dfs(root, to_string(root->val), res);
        return res;
    }

    void dfs(TreeNode* r, string path, vector<string> &ret){
        //叶子节点
        if(r->left == NULL && r->right == NULL)
            ret.push_back(path);

        if(r->left)
            dfs(r->left, path + "->" + to_string(r->left->val), ret);

        if(r->right)
            dfs(r->right, path + "->" + to_string(r->right->val), ret);
    }
};