Binary Tree Postorder Traversal @LeetCode

PROBLEM :

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].

Note: Recursive solution is trivial, could you do it iteratively?

--------------------------------------------------------------------------------
SIMPLE c++ IMPLEMENTATION : (Recursive 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<int> postorderTraversal(TreeNode* root) {
        vector<int> vec ;
        if(root==NULL)
        return vec ;
       
        postorder(root,vec) ;
        return vec ;
    }
   
    void postorder(TreeNode* root,vector<int> &vec){
    if(root==NULL)
    return ;
   
    postorder(root->left,vec) ;
    postorder(root->right,vec) ;
    vec.push_back(root->val) ;
}
};

--------------------------------------------------------------------------------
SIMPLE c++ IMPLEMENTATION : (Recursive Solution - Using Two Stacks)
--------------------------------------------------------------------------------

/**
 * 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> vec ;
        if(root==NULL)
            return vec ;
           
        stack<TreeNode*> sta1 ;
        stack<TreeNode*> sta2 ;
       
        sta1.push(root) ;
        while(!sta1.empty()){
            TreeNode* curr=sta1.top() ;
            sta1.pop() ;
           
            sta2.push(curr) ;
            if(curr->left)
                sta1.push(curr->left) ;
            if(curr->right)
                sta1.push(curr->right) ;
        }
       
        while(!sta2.empty()){
            vec.push_back((sta2.top())->val) ;
            sta2.pop() ;
        }
       
        return vec ;
    }
};

--------------------------------------------------------------------------------
SIMPLE c++ IMPLEMENTATION : (Recursive Solution - Using One Stack)
--------------------------------------------------------------------------------

/**
 * 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> vec ;
        if(root==NULL)
            return vec ;
           
        stack<TreeNode*> sta ;
        sta.push(root) ;
       
        while(!sta.empty()){
            TreeNode* curr=sta.top() ;
            sta.pop() ;
           
            vec.push_back(curr->val) ;
           
            if(curr->left)
                sta.push(curr->left) ;
            if(curr->right)
                sta.push(curr->right) ;
        }
        reverse(vec.begin(),vec.end());
        return vec ;
    }
};

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