POJ 3126, Prime Path

#include <iostream>
#include <queue>
#include <cstdio>
#include <cstring>

using namespace std;

int main()
{
//init prime table
bool prime[10001];
memset(prime,1,sizeof (prime));
prime[0] = false;prime[1] = false;
for (int i = 2; i < 5000; ++i)
{
int k = 2;
int next;
while((next = (k++) * i) < 10000)
prime[next] = false;
}

int cases;
cin >> cases;
int beg, end;
int step;
int visited[10001];
int base[4]= {1, 10, 100, 1000};
for (int c = 0; c < cases; ++c)
{
scanf("%d %d", &beg, &end);
memset(visited, -1, sizeof(visited));
queue<int> q;
q.push(beg);
visited[beg] = 0;
step = -1;

while (!q.empty())
{
int cur = q.front();
q.pop();
if (cur == end)
{
step = visited[cur];
break;
}

for (int i = 0; i < 4; ++i)
{
for (int j = 0; j < 10; ++j)
{
int l = cur / (base[i] * 10);
int r = cur % base[i];//base[i]数组很巧妙很好。值得学习
int num = l * base[i] * 10 + j * base[i] + r;
if (num > 1000 && num != cur && prime[num] == true && visited[num] == -1)
{
q.push(num);
visited[num] = visited[cur] + 1;
}
}
}
}

if (step == -1)cout << "Impossible\n";
else cout << step << endl;
}

return 0;
}