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。

 1 #include <bits/stdc++.h>
 2 typedef long long ll;
 3 using namespace std;
 4 int main()
 5 {
 6     int n,sum = 1;
 7     string s;
 8     cin >> n >> s;
 9     char last = s[0];
10     for(int i = 1 ; i < n ; i ++)
11     {
12         if(last != s[i])
13         {
14             sum ++;
15             last = s[i];
16         }
17     }
18     if(sum >= n - 1)
19         cout << n << endl;
20     else
21         cout << sum + 2 << endl;
22     return 0;
23 }