prblm145.java

import java.util.*;

public class prblm145 {
    public static void main(String[] args) {
        TreeNode root = new TreeNode(1);

        root.left = null;                 
        root.right = new TreeNode(2);     
        root.right.left = new TreeNode(3);
        root.right.right = null;

        System.out.println(new prblm145().postorderTraversal(root));
    }

    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> ans = new ArrayList<>();
        postOrder(root, ans);
        return ans;
    }

    private void postOrder(TreeNode root, List<Integer> ans){
        if(root == null) return;

        postOrder(root.left, ans);
        postOrder(root.right, ans);
        ans.add(root.val);
    }
}