CF 1196D2 RGB Substring (hard version) --- 前缀和 + 思维

简单的情况不用前缀和，直接暴力就好，数据范围大的D2，就用前缀和数组存不满足三种情况的数，最后减一下取小就可以。

#include<bits/stdc++.h> 
#define mem(a) memset(a,0,sizeof(a))
#define ll long long
#define ld long double
#define ull unsigned long long
#define mp make_pair
#define pb push_back
#define inf 0x3f3f3f3f
using namespace std;
const int N=2e5+5;
const int M=100+5;
int n,t,k,x[N],y[N],z[N];
string s;
char ss[3]={'R','G','B'};
int main()
{
    cin>>t;
    while(t--)
    {
        cin>>n>>k;
        cin>>s;
        int ans=n+5;
        x[0]=y[0]=z[0]=0;
        for(int i=0;i<n;i++)
        {
            if(s[i]!=ss[i%3]) x[i+1]=x[i]+1;
            else x[i+1]=x[i];
            if(s[i]!=ss[(i+1)%3]) y[i+1]=y[i]+1;
            else y[i+1]=y[i];
            if(s[i]!=ss[(i+2)%3]) z[i+1]=z[i]+1;
            else z[i+1]=z[i];
        }
        for(int i=0;i+k<=n;i++)
        {
            ans=min(min(ans,x[i+k]-x[i]),min(y[i+k]-y[i],z[i+k]-z[i]));
        }
        cout<<ans<<endl;
    }
}