leetcode110 Balanced Binary Tree

private int height(TreeNode node) {
        if (node == null) {
            return 0;
        } else {
            int i = height(node.left);
            int j = height(node.right);
            return (i < j) ? j + 1 : i + 1;
        }
    }

public class leetcode110 {
    public boolean isBalanced(TreeNode root) {
        int res = helper(root);
        return res != -1;
    }

    private int helper(TreeNode node) {
        if (node == null) {
            return 0;
        } else {
            int i = helper(node.left);
            int j = helper(node.right);
            if (i == -1 || j == -1)
                return -1;
            else {
                if (Math.abs(i - j) > 1) return -1;
                else
                    return Math.max(i, j) + 1;
            }
        }
    }
}