Method helper

src/com/blankj/easy/_0543/Solution.java:22–28

(TreeNode root)

    }

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

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