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[], s2[], s3[];

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

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

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

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