class Solution {
public:
    vector<int> postorderTraversal(TreeNode *root) {
        stack<TreeNode*> nodeStack;
        vector<int> result;
        //base case
        if(root==NULL)
        return result;
        nodeStack.push(root);
        while(!nodeStack.empty())
        {
            TreeNode* node= nodeStack.top();
            result.push_back(node->val);
            nodeStack.pop();
            if(node->left)
            nodeStack.push(node->left);
            if(node->right)
                nodeStack.push(node->right);
        }
        reverse(result.begin(),result.end());
        return result;
    }
};