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>ans;
    vector<string> binaryTreePaths(TreeNode* root) {
        if(root == NULL) return ans;
        string head = to_string(root->val);
        if(root->left) oneTreePath(head, root->left);
        if(root->right) oneTreePath(head, root->right);
        if(!root->left && !root->right)
        ans.push_back(head);
        return ans;
    }
    void oneTreePath(string head, TreeNode* root){
      string newHead = head + "->" + to_string(root->val);
      if(root->left) oneTreePath(newHead, root->left);
      if(root->right) oneTreePath(newHead, root->right);
      if(!root->left && !root->right)
        ans.push_back(newHead); 
    }
};