postorder

class Solution {
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);

        return;

    }

    vector<int> postorderTraversal(TreeNode* root) {
         vector<int> ans;
        postorder(root, ans);

        return ans;
    }
};