【codeforces 510B】Fox And Two Dots

【题目链接】:http://codeforces.com/contest/510/problem/B

【题意】

让你在一个二维的方格里面找环;
两个点有相邻的边它们才是相连的;
有环YES，没环NO

【题解】

写个flood fill,记录之前走的方向是哪个方向就好;
下次不要往回走;
然后如果能够碰到之前走过的点,就说明形成环了.

【Number Of WA】

0

【完整代码】

#include <bits/stdc++.h>
using namespace std;
#define lson l,m,rt<<1
#define rson m+1,r,rt<<1|1
#define LL long long
#define rep1(i,a,b) for (int i = a;i <= b;i++)
#define rep2(i,a,b) for (int i = a;i >= b;i--)
#define mp make_pair
#define ps push_back
#define fi first
#define se second
#define rei(x) cin >> x
#define pri(x) cout << x
#define ms(x,y) memset(x,y,sizeof x)

typedef pair<int,int> pii;
typedef pair<LL,LL> pll;

const int dx[9] = {0,1,0,-1,0,-1,-1,1,1};
const int dy[9] = {0,0,-1,0,1,-1,1,-1,1};
const double pi = acos(-1.0);
const int N = 60;

int n,m,a[N][N],tag[N][N],cnt,now;
char s[N];

bool dfs(int x,int y,int pre)
{
    if (x<=0 || x>=n+1||y<=0 || y>=m+1) return false;
    if (a[x][y]!=now) return false;
    if (tag[x][y]==cnt) return true;
    tag[x][y] = cnt;
    bool ok = false;
    rep1(j,1,4)
        if (j!=pre)
        {
            int tpre = j+2;
            if (tpre>4) tpre-=4;
            ok = ok|dfs(x+dx[j],y+dy[j],tpre);
        }
    return ok;
}

int main()
{
    //freopen("F:\rush.txt","r",stdin);
    ios::sync_with_stdio(false);
    rei(n),rei(m);
    rep1(i,1,n)
    {
        rei((s+1));
        rep1(j,1,m)
            a[i][j]=s[j]-'A'+1;
    }
    rep1(i,1,n)
        rep1(j,1,m)
            if (tag[i][j]==0)
            {
                now = a[i][j];
                ++cnt;
                if (dfs(i,j,-1))
                    return pri("Yes"<<endl),0;
            }
    pri("No"<<endl);
    //printf("
%.2lf sec 
", (double)clock() / CLOCKS_PER_SEC);
    return 0;
}