You are given a string letters made of N English letters. Count the number of different letters that appear in both uppercase and lowercase where all lowercase occurrences of the given letter appear before any uppercase occurrence.
For example, for letters = "aaAbcCABBc" the answer is 2. The condition is met for letters ‘a’ and ‘b’, but not for ‘c’.
Write a function:
class Solution {   public int solution(String letters)