Iteration based traverse

Iteration based preorder

public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            root = stack.pop();
            res.add(root.val);
            if (root.right != null) {
                stack.push(root.right);
            }
            if (root.left != null) {
                stack.push(root.left);
            }
        }
        return res;
    }

Iteration based inorder

public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        Stack<TreeNode> stack = new Stack<>();
        while (root != null || !stack.isEmpty()) {
            while (root != null) {
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            res.add(root.val);
            root = root.right;
            
        }

        return res;

    }

Iteration based postorder

public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) {
            return res;
        }

        Stack<TreeNode> stack = new Stack<>();
        stack.add(root);
        while (!stack.isEmpty()) {
            root = stack.pop();
            res.add(0, root.val);
            if (root.left != null) {
                stack.add(root.left);
            }
            if (root.right != null) {
                stack.add(root.right);
            }
        }
        return res;
    }