HDU1973 http://acm.hdu.edu.cn/showproblem.php?pid=1973

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<queue>
#include<math.h>
#define N 10010

using namespace std;

int vis[N];
char s1[5], s2[5], s3[5];

struct node
{
    int step;
    char st[5];
};

int prime(int y)
{
    int i, k = (int)sqrt(y);
    for(i = 2 ; i <= k ; i++)
        if(y % i == 0)
            return 0;
    return 1;
}

int BFS(char s[])
{
    queue<node>Q;
    node now, next;
    int i, j, h, x = 0;
    memset(vis, 0, sizeof(vis));
    memset(s3, 0, sizeof(s3));
    for(i = 0 ; i < 4 ; i++)
        x = x * 10 + (s[i] - '0');
    vis[x] = 1;
    strcpy(now.st, s);
    now.step = 0;
    Q.push(now);
    while(!Q.empty())
    {
        now = Q.front();
        Q.pop();
        if(strcmp(now.st, s2) == 0)
            return now.step;
        for(i = 0 ; i < 4 ; i++)//各个位（即个位，十位，百位，千位）
        {
            for(j = 0 ; j < 10 ; j++)//0到9十个数
            {
                if(i == 0 && j == 0)//千位不为0
                    continue;
                if(now.st[i] == j + '0')//原有的数不需要替换
                    continue;
                strcpy(s3, now.st);
                now.st[i] = j + '0';//0到9其中的一个数去替换四位数气中的一位
                x = 0;
                for(h = 0 ; h < 4 ; h++)
                    x = x * 10 + (now.st[h] - '0');
                if(prime(x) == 1 && !vis[x])
                {
                    vis[x] = 1;
                    next.step = now.step + 1;
                    strcpy(next.st, now.st);
                    i = 0;/***/
                    j = 0;/***/
                    Q.push(next);
                }
                strcpy(now.st, s3);
            }
        }
    }
    return -1;
}

int main()
{
    int t;
    scanf("%d", &t);
    while(t--)
    {
        scanf("%s%s", s1, s2);
        printf("%d\n", BFS(s1));
    }
    return 0;
}