Codeforces 603A Alternative Thinking

题意：给你一个01串，必须替换一次，且替换的为子串。问换完后，最大01串长度。

 #include <bits/stdc++.h>
 typedef long long ll;
 using namespace std;
 int main()
 {
     int n,sum = ;
     string s;
     cin >> n >> s;
     char last = s[];
     for(int i =  ; i < n ; i ++)
     {
         if(last != s[i])
         {
             sum ++;
             last = s[i];
         }
     }
     if(sum >= n - )
         cout << n << endl;
     else
         cout << sum +  << endl;
     return ;
 }