https://blog.csdn.net/csyifanZhang/article/details/106391657

推导过程↑

#define ll long long
#define vec vector<int>
#define inf 0x3f3f3f3f
#define MAX 100005
#define P pair<ll,ll>
#define MOD 1000000

int main() {
	string s;
	int s1[MAX], s2[MAX], s3[MAX];//到了第i个位置，一共出现了s[i]次R/G/B
	while (cin >> s) {
		memset(s1, 0, sizeof(s1));
		memset(s2, 0, sizeof(s2));
		memset(s3, 0, sizeof(s3));
		int len = s.size(), res = inf;
		for (int i = 0; i < len; i++) {
			int a = 0, b = 0, c = 0;
			if (s[i] == 'R')a++;
			else if (s[i] == 'G')b++;
			else c++;
			s1[i + 1] = s1[i] + a;
			s2[i + 1] = s2[i] + b;
			s3[i + 1] = s3[i] + c;
		}
		//通过i，j将串分成三份，第一段保留R，第二段保留G，第三段保留B
		for (int i = 0; i <= len; i++) {
			for (int j = i; j <= len; j++) {
				int cnt = s1[i] + s2[j] - s2[i] + s3[len] - s3[j];
				if (len - cnt < res)
					res = len - cnt;
			}
		}
		cout << res << endl;
	}
}