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 = 1;
    string s;
    cin >> n >> s;
    char last = s[0];
    for(int i = 1 ; i < n ; i ++)
    {
        if(last != s[i])
        {
            sum ++;
            last = s[i];
        }
    }
    if(sum >= n - 1)
        cout << n << endl;
    else
        cout << sum + 2 << endl;
    return 0;
}