postorder

import java.util.*;

class Solution {

  /**
   * @param tree
   *     The input BinaryTree.
   * @return A list of all keys in the tree, in post-order.
   */
  public static List<Integer> postOrder(BinaryTree tree) {
    // TODO
    if(tree == null){
      return new ArrayList<>();
    }
    List<Integer> list = new ArrayList<>();
     checkLeft(list, tree);
     checkRight(list, tree);
     list.add(tree.getKey());
    return list;

  }
  public static void checkLeft(List<Integer> list, BinaryTree tree ){
    if(tree.hasLeft()){
      list.addAll(postOrder(tree.getLeft()));
    }
   }
 public static void checkRight(List<Integer> list, BinaryTree tree ){
    if(tree.hasRight()){
      list.addAll(postOrder(tree.getRight()));
    }
   }
}