145. Binary tree PostOrder Traversal

public:
    void postorder(TreeNode* root,vector<int> &ans){
        if(root == NULL) return;
        postorder(root->left,ans);
        postorder(root->right,ans);
        ans.push_back(root->val);
    }
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int>ans;
        postorder(root,ans);
        return ans;
    }
};