/** * Title : A Famous Grid * url: http://acm.hdu.edu.cn/showproblem.php?pid=4255 * Time: 2012-7-26- 19:00 around * stratege: BFS, Prime Number * Author: Johnsondu * Status: 63403822012-07-26 19:23:33Accepted42551718MS6260K2349 B*/#include <iostream>#include <cstdio>#include <cmath>#include <cstring>#include <algorithm>#include <queue>using namespace std ;const int MAXN = 1000005 ;int mat[1005][1005] ;bool mark[1005][1005], flag, prime[MAXN] ;int sVal, dVal ;int sx, sy, ans ;int dx[4] = {0, 0, 1, -1} ;int dy[4] = {1, -1, 0, 0} ;struct Node{int x, y ;int step ;};Node cur, next ;void Graph () //蛇形填数 制图{int x, y, z, b, i, c = 1000000 ;x = 1; y = 1000; z = 1; b = 1000 ;while (c > 0){for (i = x; i <= y; i ++)mat[x][i] = c -- ;x ++ ;for (i = x; i <= y; i ++)mat[i][y] = c -- ;y -- ;for (i = y; i >= z; i --)mat[b][i] = c -- ;b -- ;for (i = b; i > z; i --)mat[i][z] = c -- ;z ++ ;}} void IsNotPrime () // 筛选1000000内的素数{int i, tmp ;memset (prime, true, sizeof (prime)) ;prime[0] = prime[1] = false ;for (i = 4; i < MAXN; i += 2)prime[i] = false ;for (i = 3; i < MAXN; i += 2){if (prime[i]){tmp = 2 * i ;while (tmp < MAXN){prime[tmp] = false ;tmp += i ;}}}}void init () //初始化 并找出初始位置{int i, j ;memset (mark, false, sizeof (mark)) ;flag = false ;ans = 0 ;for (i = 1; i < 1001; i ++)for (j = 1; j < 1001; j ++){if (mat[i][j] == sVal){sx = i ;sy = j ;return ;}}}bool safe (int x, int y) //判断是否可行： 越界，标记与否，素数{if (x < 1001 && y < 1001 && x > 0 && y > 0 && !mark[x][y] && !prime[mat[x][y]])return true ;return false ;}void bfs () //常规的bfs部分{int i ;cur.x = sx ;cur.y = sy ;cur.step = 0 ;mark[sx][sy] = true ;queue <Node> Q ;Q.push (cur) ;while (!Q.empty ()){cur = Q.front () ;Q.pop () ;for (i = 0; i < 4; i ++){next.x = cur.x + dx[i] ;next.y = cur.y + dy[i] ;if (safe (next.x, next.y)){next.step = cur.step + 1; if (mat[next.x][next.y] == dVal){ans = next.step ;flag = true ;return ;}mark[next.x][next.y] = true ;Q.push (next) ;}}}}int main (){int cas = 1; Graph () ;IsNotPrime () ;while (scanf ("%d%d", &sVal, &dVal) != EOF){init () ;bfs () ;printf ("Case %d: ", cas ++) ;if (flag)printf ("%d\n", ans) ;elseprintf ("impossible\n") ;}return 0 ;}
