Method helper

src/com/blankj/easy/_0110/Solution.java:19–27

(TreeNode node)

    }

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

    public static void main(String[] args) {
        Solution solution = new Solution();