[HackerRank][Python3] Two Characters

Problem :

https://www.hackerrank.com/challenges/two-characters/problem

My Solution :

#!/usr/bin/env python3

def is_alternating(a, b, s):
    t = 0
    for c in s:
        if c in (a, b):
            if c == t:
                return False
            t = c
    return True

def twoCharaters(s):
    cnt_c = {}
    for c in s:
        cnt_c.setdefault(c, 0)
        cnt_c[c] += 1
    sorted_keys = sorted(cnt_c, key=cnt_c.get, reverse=True)
    for i in range(len(sorted_keys)-1):
        for j in range(i+1, len(sorted_keys)):
            if cnt_c[sorted_keys[i]] - cnt_c[sorted_keys[j]] > 1:
                break
            if is_alternating(sorted_keys[i], sorted_keys[j], s):
                return cnt_c[sorted_keys[i]] + cnt_c[sorted_keys[j]]
    return 0


l = int(input())
s = input().strip()
result = twoCharaters(s)
print(result)