Symmetric Tree

Given a binary tree, check whether it is a mirror of itself.

• Time: O(N)
• Space: O(1)

    public boolean isSymmetric(TreeNode root) {
        if (root == null) {
            return true;
        }
        return isSymmetric(root.left, root.right);
    }

    private boolean isSymmetric(TreeNode left, TreeNode right) {
        if (left == null && right == null) {
            return true;
        }
        if (left == null || right == null) {
            return false;
        }
        return left.val == right.val && 
                isSymmetric(left.left, right.right) && 
                isSymmetric(left.right, right.left);
    }