// TODO: Add more effective stack based solution

public List<Integer> postorderTraversal(TreeNode root) {

List<Integer> res = new ArrayList<Integer>();

if (root == null) return res;

Map<TreeNode, Boolean> hasChilren = new HashMap<>();

Stack<TreeNode> s = new Stack<>();

s.add(root);

while (!s.isEmpty()) {

TreeNode curr = s.peek();

if ((curr.left == null && curr.right == null) ^ !hasChilren.getOrDefault(curr, true)) {

res.add(s.pop().val);

continue;

}

if (curr.right != null)

s.add(curr.right);

if(curr.left != null)

s.add(curr.left);

hasChilren.put(curr, false);

}

return res;

}

@SuppressWarnings("unused")

private void postOrd(TreeNode root) {

if (root == null) return;

postOrd(root.left);

postOrd(root.right);

System.out.println(root.val);

}

public static void main(String[] args) {

BinaryTreePostorderTraversal sln = new BinaryTreePostorderTraversal();

TreeNode t1 = initTree(1, 0, 4, 2, 3, 5, null);

TreeNode t2 = initTree(5, 3, 7, 1, 4, 6, 8);

System.out.println(sln.postorderTraversal(t1));

System.out.println(sln.postorderTraversal(t2));

System.out.println(sln.postorderTraversal(null));

System.out.println(sln.postorderTraversal(initTree(1)));

}