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"]

Solution:

/**
 * 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 ) return res;
        vector<string> res1 = binaryTreePaths(root->left);
        vector<string> res2 = binaryTreePaths(root->right);
        int n1 = res1.size(), n2 = res2.size();
        if ( n1 == 0 and n2 == 0 ) res.push_back(to_string(root->val));
        if ( n1 != 0 ) {
            for ( int i = 0; i <= n1-1; i++ ) {
                res.push_back(to_string(root->val) + "->" + res1[i]);
            }
        }
        if ( n2 != 0 ) {
            for ( int i = 0; i <= n2-1; i++ ) {
                res.push_back(to_string(root->val) + "->" + res2[i]);
            }
        }
        return res;
    }
};

Notes
Note the special case of leaf node!