Binary Tree Paths

C++代码：
vector<string> binaryTreePaths(TreeNode* root) {    
vector<string> ans;    
if (!root) return ans;    
vector<string> left = binaryTreePaths(root->left), right = binaryTreePaths(root->right);
ans.insert(ans.end(), left.begin(), left.end());
ans.insert(ans.end(), right.begin(), right.end());    
if (!ans.size()) ans.push_back(to_string(root->val));    
else {        
          for (string &s : ans) {
            s = to_string(root->val) + "->" + s;
          }
     }    
     return ans;
}

题目如下：

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"]