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

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

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> ret = new LinkedList<>();
        Stack<TreeNode> stack = new Stack<>();
        TreeNode cur = root;
        while(cur != null || !stack.isEmpty()) {
            if(cur != null) {
                ret.add(0, cur.val);
                stack.push(cur);
                cur = cur.right;
            } else {
                cur = stack.pop();
                cur = cur.left;       
            }
        }
        return ret;
    }
}