Postorder Traversal (Iterative)

//--------using one stack--------

vector<int> postOrder(Node* node) {
    if(!node)return {};
    stack<Node*>s;
    vector<int>v;
    while(!s.empty()||node){
        if(node){
            v.push_back(node->data);
            s.push(node);
            node=node->right;
        }
        else{
            node=s.top()->left;
            s.pop();
        }
    }
    reverse(v.begin(),v.end());
    return v;
}


//--------using two stack--------
class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        if(!root)return {};
        stack<TreeNode*>s1,s2;
        vector<int>v;
        s2.push(root);
        while(!s2.empty()){
            TreeNode*ll=s2.top();
            s2.pop();
            s1.push(ll);
            if(ll->left)s2.push(ll->left);
            if(ll->right)s2.push(ll->right);
        }
        while(!s1.empty()){
            v.push_back(s1.top()->val);
            s1.pop();
        }
        return v;
    }
};