枚举 K 然后暴力翻转检验

#include <bits/stdc++.h>
using namespace std;
int n;
string s, t;
int check(int k)
{
    int cnt = 0;
    for (int i = 0; i + k - 1 < t.size(); i++)
        if (t[i] == 'B')
        {
            cnt++;
            for (int j = i; j <= i + k - 1; j++)
            {
                if (t[j] == 'B')
                    t[j] = 'F';
                else
                    t[j] = 'B';
            }
        }
    for (int i = (int)t.size() - k; i < t.size(); i++)
        if (t[i] == 'B')
            return -1;
    return cnt;
}
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cin >> n;
    s = "";
    for (int i = 1; i <= n; i++)
    {
        char x;
        cin >> x;
        s += x;
    }
    int ansCnt = n, ansK = 1;
    for (int k = 1; k <= n; k++)
    {
        t = s;
        int cnt = check(k);
        if (cnt != -1 && cnt < ansCnt)
        {
            ansCnt = cnt;
            ansK = k;
        }
    }
    cout << ansK << " " << ansCnt << "\n";
    return 0;
}

差分数组加速翻转

#include <bits/stdc++.h>
using namespace std;
int n;
string s, t;
int d[5005];
int check(int k)
{
    for (int i = 0; i <= n; i++)
        d[i] = 0;
    int cnt = 0;
    int now = 0; // 差分之和
    int i;
    for (i = 0; i + k - 1 < t.size(); i++)
    {
        now += d[i];
        if (t[i] == 'B' && now % 2 == 0 ||
            t[i] == 'F' && now % 2 == 1)
        {
            cnt++;
            now++;
            d[i + k]--;
        }
    }
    for (; i < t.size(); i++)
    {
        now += d[i];
        if (t[i] == 'B' && now % 2 == 0 ||
            t[i] == 'F' && now % 2 == 1)
            return -1;
    }
    return cnt;
}
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cin >> n;
    s = "";
    for (int i = 1; i <= n; i++)
    {
        char x;
        cin >> x;
        s += x;
    }
    int ansCnt = n, ansK = 1;
    for (int k = 1; k <= n; k++)
    {
        t = s;
        int cnt = check(k);
        if (cnt != -1 && cnt < ansCnt)
        {
            ansCnt = cnt;
            ansK = k;
        }
    }
    cout << ansK << " " << ansCnt << "\n";
    return 0;
}