Binary Tree Postorder Traversal

Given a binary tree, return the postorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

   1
    \
     2
    /
   3

return [3,2,1].

 /**
  * 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<int> postorderTraversal(TreeNode* root) {
         vector<int> res;
         help(root,res);
         return res;
     }
     void help(TreeNode *root,vector<int> &res)
     {
         if(root!=NULL)
         {
             help(root->left,res);
             help(root->right,res);
             res.push_back(root->val);
         }
     }
 };
 class Solution {
 public:
     vector<int> postorderTraversal(TreeNode *root)
     {
         vector<int> res;
         stack<pair<TreeNode*,int>> s;
         s.push(make_pair(root,));
         while(!s.empty())
         {
             TreeNode *now=s.top().first;
             if(now==NULL)
             s.pop();
             else
             {
                 switch(s.top().second++)
                 {
                     case :
                         s.push(make_pair(now->left,));
                         break;
                     case :
                         s.push(make_pair(now->right,));
                         break;
                     default:
                         s.pop();

                         res.push_back(now->val);
                         break;
                 }
             }
         }
         return res;
     }
 };