public boolean isSymmetricIter(TreeNode root) {
        if(root == null) return true;
        Stack<TreeNode> leftStack = new Stack<TreeNode>();
        Stack<TreeNode> rightStack = new Stack<TreeNode>();
        leftStack.push(root.left);
        rightStack.push(root.right);
        while (leftStack.size() > 0 && rightStack.size() > 0){
            TreeNode left = leftStack.pop();
            TreeNode right = rightStack.pop();
            if(left == null && right == null) continue;
            if(left == null || right == null) return false;
            if(left.val == right.val) {
                leftStack.push(left.right);
                leftStack.push(left.left);
                rightStack.push(right.left);
                rightStack.push(right.right);
            }else{
                return false;
            }
        }
        return true;
    }