http://codeforces.com/contest/196/problem/B
We've got a rectangular n × m-cell maze. Each cell is either passable, or is a wall (impassable). A little boy found the maze
and cyclically tiled a plane with it so that the plane became an infinite maze. Now on this plane cell (x, y) is a wall if
and only if cell 
is
a wall.
In this problem 
is
a remainder of dividing number a by number b.
The little boy stood at some cell on the plane and he wondered whether he can walk infinitely far away from his starting position. From cell (x, y) he can go to one of the following cells: (x, y - 1), (x, y + 1), (x - 1, y) and (x + 1, y), provided that the cell he goes to is not a wall.
The first line contains two space-separated integers n and m (1 ≤ n, m ≤ 1500) — the height and the width of the maze that the boy used to cyclically tile the plane.
Each of the next n lines contains m characters — the description of the labyrinth. Each character is either a "#", that marks a wall, a ".", that marks a passable cell, or an "S", that marks the little boy's starting point.
The starting point is a passable cell. It is guaranteed that character "S" occurs exactly once in the input.
Print "Yes" (without the quotes), if the little boy can walk infinitely far from the starting point. Otherwise, print "No" (without the quotes).
5 4 ##.# ##S# #..# #.## #..#
Yes
5 4 ##.# ##S# #..# ..#. #.##
No
In the first sample the little boy can go up for infinitely long as there is a "clear path" that goes vertically. He just needs to repeat the following steps infinitely: up, up, left, up, up, right, up.
In the second sample the vertical path is blocked. The path to the left doesn't work, too — the next "copy" of the maze traps the boy.
题意：这个迷宫是可以上下左右无限拼接的，问从S点出发是不是能走到无限远。
一开始，自己写的bfs，wa4和mle6，很尴尬，看了别人写的题解才摸索出来
思路：要从S能走到无限远，其实就是S本身能到某个点，然后这条路在x>=n||x<0||y>=m||y<0取模以后又可以到达这个点，即第二次到达，就算联通了，能无限走了。
#include<bits/stdc++.h>
using namespace std;
char s[1505][1505];
int f[1505][1505];
struct node
{
int x,y;
} st,vis[1505][1505];//vis 用来记录第一次到这个点时的座标，然后比较第二次来的时候的座标
int xx[4]= {0,0,1,-1};
int yy[4]= {1,-1,0,0};
int n,m;
int bfs()
{
node c;
f[st.x][st.y]=1;
vis[st.x][st.y].x=st.x;
vis[st.x][st.y].y=st.y;
queue<node > q;
q.push(st);
while(!q.empty())
{
node d=q.front();
q.pop();
for(int i=0; i<4; i++)
{
int x=d.x+xx[i];
int y=d.y+yy[i];
node cc;
cc.x=x;
cc.y=y;
c.x=(x%n+n)%n;
c.y=(y%m+m)%m;
if(s[c.x][c.y]=='#') continue;
if(!f[c.x][c.y])
{
f[c.x][c.y]=1;
q.push(cc);//注意是cc
vis[c.x][c.y].x=x;
vis[c.x][c.y].y=y;
}
else
{
if((vis[c.x][c.y].x!=x)||(vis[c.x][c.y].y!=y))//第二次到的座标与第一次不同，true
{
return 1;
}
}
}
}
return 0;
}
int main()
{
scanf("%d%d",&n,&m);
for(int i=0; i<n; i++)
{
scanf("%s",s[i]);
for(int j=0; j<m; j++)
{
if(s[i][j]=='S')
{
st.x=i;
st.y=j;
s[i][j]='.';
}
}
}
if(bfs())
{
puts("Yes");
}
else
{
puts("No");
}
return 0;
}