Binary Tree Postorder Traversal using 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;
    }
};