cf#301-C. Ice Cave-BFS

http://codeforces.com/contest/540/problem/C

题意 给n*m的地图

只能走.不能走X，并且点.被走了之后会变成X，问能否从点1走到点2，并且要求最后点2要是X（为了掉到下一level）   

因此直接bfs搜，每次走过 点 后把 点变成X ，最后判断能否到达点2 并且点2状态为 X即可

另一种是直接判断能不能到点2，然后如果点2本身是X则输yes，

如果是点的话，只需要看能否往外走一步然后退回来即可

即看四周有没一个合法的点，

合法的点首先要是【点】，其次没被访问过，如果被访问过 【访问的步数必须==到达点2的步数-1】 

1:

#include <cstdio>#include <cmath>#include <cstring>#include <string>#include <algorithm>#include <queue>#include <map>#include <set>#include <vector>#include <iostream>using namespace std;const double pi=acos(-1.0);double eps=0.000001; __int64 min(__int64 a,__int64 b){return a<b? a:b ;} __int64 max(__int64 a,__int64 b) {return a>b?a:b;}int n,m;char mp[505][505];int vis[505][505];int dx[]={0,0,1,-1};int dy[]={1,-1,0,0};struct node{int x,y;int step;node(){}node(int a,int b,int c){x=a,y=b;step=c;}};queue<node> q;node st,ed;int main(){ int i;cin>>n>>m;int x,y;for (i=1;i<=n;i++){scanf("%s",mp[i]+1);}scanf("%d%d",&st.x,&st.y);scanf("%d%d",&ed.x,&ed.y);int flag=0;st.step=0;q.push(st);while(!q.empty()){node tp=q.front();q.pop();for (i=0;i<4;i++){int x=tp.x+dx[i];int y=tp.y+dy[i];if (x<1||x>n||y<1||y>m) continue;//if (vis[x][y])continue;if (x==ed.x&&y==ed.y&&mp[x][y]=='X'){flag=tp.step+1;break;}if (mp[x][y]=='X')continue;mp[x][y]='X';//vis[x][y]=tp.step+1;q.push(node(x,y,tp.step+1));}if (flag)break; }if(flag){  printf("YES\n"); }else{printf("NO\n");}return 0; }

2:

#include <cstdio>#include <cmath>#include <cstring>#include <string>#include <algorithm>#include <queue>#include <map>#include <set>#include <vector>#include <iostream>using namespace std;const double pi=acos(-1.0);double eps=0.000001; __int64 min(__int64 a,__int64 b){return a<b? a:b ;} __int64 max(__int64 a,__int64 b) {return a>b?a:b;}int n,m;char mp[505][505];int vis[505][505];int dx[]={0,0,1,-1};int dy[]={1,-1,0,0};struct node{int x,y;int step;node(){}node(int a,int b,int c){x=a,y=b;step=c;}};queue<node> q;node st,ed;int main(){ int i;cin>>n>>m;int x,y;for (i=1;i<=n;i++){scanf("%s",mp[i]+1);}scanf("%d%d",&st.x,&st.y);scanf("%d%d",&ed.x,&ed.y);int flag=0;st.step=0;q.push(st);int lastx,lasty;while(!q.empty()){node tp=q.front();q.pop();for (i=0;i<4;i++){int x=tp.x+dx[i];int y=tp.y+dy[i];if (x<1||x>n||y<1||y>m) continue;if (vis[x][y])continue;if (x==ed.x&&y==ed.y){flag=tp.step+1;lastx=tp.x,lasty=tp.y;break;}if (mp[x][y]=='X')continue;vis[x][y]=tp.step+1;q.push(node(x,y,tp.step+1));}if (flag)break; }if(flag){if (mp[ed.x][ed.y]=='X') printf("YES\n");else{int cun=0;for (i=0;i<4;i++){int x=ed.x+dx[i];int y=ed.y+dy[i];if (x<1||x>n||y<1||y>m) continue; if (mp[x][y]=='X')continue; if ((vis[x][y]&&vis[x][y]<flag-1))continue; if (x==lastx&&y==lasty) continue;cun++;}if (cun>=1) printf("YES\n");elseprintf("NO\n");}}else{printf("NO\n");}return 0; }