直接从 B 位置开始 Floodfill 即可。如果 Floodfill 到了 L 位置，那么答案为 Yes，否则为 No。

#include <bits/stdc++.h>
using namespace std;
const int K = 1000 + 12;
char x[K][K];
int main()
{
	freopen("honey1.in", "r", stdin);
	freopen("honey1.out", "w", stdout);
	int T;
	cin >> T;
	while (T--)
	{
		int k, sx = 0, sy = 0;
		cin >> k;
		for (int i = 1; i <= k; i++)
			for (int j = 1; j <= k; j++)
			{
				cin >> x[i][j];
				if (x[i][j] == 'B') sx = i, sy = j;
			}
		for (int i = 0; i <= k + 1; i++)
			x[0][i] = x[i][0] = x[i][k + 1] = x[k + 1][i] = 'X';
		queue<pair<int, int> > q;
		q.push(make_pair(sx, sy));
		x[sx][sy] = 'X';
		bool ok = false;
		while (!q.empty())
		{
			pair<int, int> u = q.front();
			q.pop();
			if (x[u.first][u.second + 1] == 'L')
			{
				ok = true;
				puts("Yes");
				break;
			}
			if (x[u.first][u.second + 1] != 'X')
			{
				x[u.first][u.second + 1] = 'X';
				q.push(make_pair(u.first, u.second + 1));
			}
			if (x[u.first][u.second - 1] == 'L')
			{
				ok = true;
				puts("Yes");
				break;
			}
			if (x[u.first][u.second - 1] != 'X')
			{
				x[u.first][u.second - 1] = 'X';
				q.push(make_pair(u.first, u.second - 1));
			}
			if (x[u.first + 1][u.second] == 'L')
			{
				ok = true;
				puts("Yes");
				break;
			}
			if (x[u.first + 1][u.second] != 'X')
			{
				x[u.first + 1][u.second] = 'X';
				q.push(make_pair(u.first + 1, u.second));
			}
			if (x[u.first - 1][u.second] == 'L')
			{
				ok = true;
				puts("Yes");
				break;
			}
			if (x[u.first - 1][u.second] != 'X')
			{
				x[u.first - 1][u.second] = 'X';
				q.push(make_pair(u.first - 1, u.second));
			}
		}
		if (!ok) puts("No");
	}
	return 0;
}

食用性更强的 DFS 代码！

#include<bits/stdc++.h>
using namespace std;
const long long maxn=1e3+5;
long long T,k,i,j,bx,by;
char ch[maxn][maxn];
long long vis[maxn][maxn],dx[]={1,-1,0,0},dy[]={0,0,1,-1};
bool flag;
void dfs(long long x,long long y)
{
	if(vis[x][y]) return;
	vis[x][y]=1;
	if(flag==true) return;
	if(ch[x][y]=='L')
	{
		flag=true;
		return;
	}
	for(long long it=0;it<4;it++)
	{
		long long hx=x+dx[it],hy=y+dy[it];
		if(hx>k || hx<1 || hy>k || hy<1 || vis[hx][hy] || ch[hx][hy]=='X') continue;
		//cout<<hx<<" "<<hy<<"\n";
		dfs(hx,hy);
	}
	return;
}
int main()
{
	freopen("honey1.in","r",stdin);
	freopen("honey1.out","w",stdout);
	ios::sync_with_stdio(false);
	cin>>T;
	while(T--)
	{
		flag=false;
		cin>>k;
		for(i=1;i<=k;i++)
		{
			for(j=1;j<=k;j++)
			{
				cin>>ch[i][j];
				if(ch[i][j]=='B') bx=i,by=j;
				vis[i][j]=0;
			}
		}
		//cout<<bx<<" "<<by<<"\n";
		dfs(bx,by);
		if(flag) puts("Yes");
		else puts("No");
	}
	return 0;
}
//Sleeping Cup can be better!
//CSP2024RP+=INF!