Postorder

class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> ans;

        while(root!=nullptr){
            ans.push_back(root->val);
            if(root->right == nullptr){
                root = root->left;
            }
            else{
                TreeNode* next = root->right;
                while(next->left!=nullptr){
                    next = next->left;
                }
                next->left = root->left;
                TreeNode* temp = root;
                root = root->right;
                temp->left=nullptr;

            }
        }

        reverse(ans.begin(), ans.end());
        return ans;
    }
};