pku 3126 bfs

#include <iostream>using namespace std;int n1, n2;bool map[10000];int value[10000];int q[11000];int c[4]= {1, 10, 100, 1000};bool bfs(){int front = 0, rear = 0;q[rear++] = n1;value[n1] = 0;while(front < rear){int temp = q[front++];if(temp == n2)return true;for(int i = 0; i < 4; ++i){int digit = (temp/c[i]) % 10;for(int j = 0; j < 10; ++j){int changed = temp - digit*c[i] + j*c[i]; //注意判断: changed >= 1000 if(changed >= 1000 && j != digit && map[changed] && value[changed] == -1){q[rear++] = changed;value[changed] = value[temp]+1;}}}}return false;}int main(){int n;for(int i = 1; i <= 9999; ++i)map[i] = true;for(int i = 2; i < 5000; ++i)if(map[i]){for(int j = 2*i; j <= 9999; j += i)map[j] = false;}scanf("%d", &n);while(n--){scanf("%d%d", &n1, &n2);memset(value, -1, sizeof(value));if(map[n1] && map[n2]){if(bfs())printf("%d/n", value[n2]);elseprintf("Impossible/n");}elseprintf("Impossible/n");}return 0;}//bfs的运用,深搜,一次搞定//注意判断: changed >= 1000