xjtuoj 1260：初学围棋

#include<bits/stdc++.h>
#define F(i,n,m) for(int i=n;i<m;i++)
typedef unsigned long long ull;
typedef long long ll;
using namespace std;
inline int read() {
	int num = 0;
	char c;
	bool flag = false;
	while ((c = getchar()) == ' ' || c == '\n' || c == '\r');
	if (c == '-') flag = true;
	else
		num = c - '0';
	while (isdigit(c = getchar()))
		num = num * 10 + c - '0';
	return (flag ? -1 : 1) * num;
}
int n;
string s1;
int sl(int k) {
	if(k==0) return 0;
	if(s1[k-1]=='W') return 3;
	if(s1[k-1]=='.') {
		if(k==1||s1[k-2]=='.') return 1;
		for(int i=k-2; i>=0; i--) {
			if(s1[i]=='.') return 1;
			else if(s1[i]=='B') return 0;
		}
	}
	return 0;
}
int sr(int k) {
	if(k==n-1) return 0;
	if(s1[k+1]=='W') return 3;
	if(s1[k+1]=='.') {
		if(k==n-2||s1[k+2]=='.') return 1;
		for(int i=k+2; i<n; i++) {
			if(s1[i]=='.') return 1;
			else if(s1[i]=='B') return 0;
		}
	}
	return 0;
}
int main() {
	std::ios::sync_with_stdio(false);
	cin>>n>>s1;
	int l,r,ans,flag;
	l=r=ans=0;
	flag=0;
	F(i,0,n) {
		if(s1[i]=='B') {
			l=i;
			flag+=sl(l);
			while(s1[i]=='B'&&i<n)i++;
			if(i==n) break; 
			r=i-1;
			flag+=sr(i-1);
//			cout<<l<<" "<<r<<" "<<flag<<" "<<sl(l)<<" "<<sr(r)<<endl;
			if(flag==4) ans=max(ans,r-l+1);
			flag=0;
			l=0;
		}
	}
	cout<<ans;
	return 0;
}