Two characters - WA)solution

//https://www.hackerrank.com/challenges/two-characters/problem
// Wrong answer - How can I fix this?

#include <bits/stdc++.h>

using namespace std;

bool check(string s){
    for(int i=1;i<s.size();i++)
        if(s[i] == s[i-1])
            return false;
    return true;
}
int main() {

    string S;
    cin >> S;

    bool ap[300];
    memset(ap,false,sizeof ap);

    int diff_letters = 0;
    for(int i=0;i<S.size();i++)
        if(!ap[ S[i]-'a' ]){
            ap[ S[i]-'a' ] = true;
            diff_letters++;
        }

    //   cout << diff_letters << endl;
    if(diff_letters == 1){
        printf("0\n");
        return 0;
    }

    int ans=0;
    for(int i=0; i<26; i++){
    for(int j=0; j<26; j++){
        if( (ap[i] && ap[j]) && (i != j) ){
            //  cout << i << " " << j << endl;

            string new_s = "";
            for(int k=0; k<S.size(); k++)
                if( (S[k] == i+'a') || (S[k] == j+'a') )
                    new_s += S[k];

            if(check(new_s)){
                int tam = new_s.size();
                ans = max(ans,tam);
            }
        }
    }}
    printf("%d\n",ans);
  return 0;
}