class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<Integer>();
        preorder(root,result);
        return result;
    }

    public void preorder(TreeNode root, List<Integer> result) {
        if (root == null){
            return;
        }
        result.add(root.val);
        preorder(root.left, result);
        preorder(root.right,result);

    }
}

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

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

    public void postOrder(TreeNode root, List<Integer> result) {
        if (root == null) {
            return;
        }
        postOrder(root.left,result);
        postOrder(root.right,result);
        result.add(root.val);
    }
}

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        List<Integer> result = new ArrayList<>();
        if (root == null ){
            return result;
        }
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            result.add(node.val);
            if (node.left!= null) {
                stack.push(node.left);
            }
            if (node.right != null){
                stack.push(node.right);
            }
        }
        Collections.reverse(result);
        return result;
       
    }
}

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<Integer>();
        inOrder(root, result);
        return result;
    }

    public void inOrder(TreeNode root, List<Integer> result) {
        if (root == null) {
            return;
        }
        inOrder(root.left, result);
        result.add(root.val);
        inOrder(root.right, result);
    }
}

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