题目大意:从一个素数变换到另一个素数最小需要多少步。
思路:将所有1000~10000的素数通过打表表示出来,然后依次枚举与s相差一位的数并标记,通过BFS找寻最小步数。
CODE:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <queue>
using namespace std;
int prime[10001];
int vis[10001];
int flag[10001];
int tot;
int s, e;
struct node
{
int x, step;
};
void init()
{
tot = 0;
for(int i = 2; i <= 10000; i++) if(!vis[i])
{
for(int j = i*i; j <= 10000; j += i)
vis[j] = 1;
}
for(int i = 1000; i <= 10000; i++) if(!vis[i])
prime[tot++] = i;
}
int check(int x, int y)
{
int cnt = 0;
while(x)
{
int xx = x%10;
int yy = y%10;
if(xx != yy) cnt++;
x /= 10;
y /= 10;
}
if(cnt == 1) return 1;
return 0;
}
int bfs()
{
queue<node> Q;
node p, q;
p.x = s;
flag[s] = 1;
p.step = 0;
Q.push(p);
while(!Q.empty())
{
p = Q.front();
Q.pop();
if(p.x == e)
{
printf("%d\n", p.step);
return 1;
}
for(int i = 0; i < tot; i++)
{
q = p;
q.x = prime[i];
if(check(p.x, q.x) && !flag[q.x])
{
q.step++;
flag[q.x] = 1;
Q.push(q);
}
}
}
return 0;
}
int main()
{
init();
int T;
scanf("%d", &T);
while(T--)
{
memset(flag, 0, sizeof(flag));
scanf("%d%d", &s, &e);
if(s == e)
{
printf("0\n");
continue;
}
int t = bfs();
if(!t) printf("impossible\n");
}
}
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <queue>
using namespace std;
int prime[10001];
int vis[10001];
int flag[10001];
int tot;
int s, e;
struct node
{
int x, step;
};
void init()
{
tot = 0;
for(int i = 2; i <= 10000; i++) if(!vis[i])
{
for(int j = i*i; j <= 10000; j += i)
vis[j] = 1;
}
for(int i = 1000; i <= 10000; i++) if(!vis[i])
prime[tot++] = i;
}
int check(int x, int y)
{
int cnt = 0;
while(x)
{
int xx = x%10;
int yy = y%10;
if(xx != yy) cnt++;
x /= 10;
y /= 10;
}
if(cnt == 1) return 1;
return 0;
}
int bfs()
{
queue<node> Q;
node p, q;
p.x = s;
flag[s] = 1;
p.step = 0;
Q.push(p);
while(!Q.empty())
{
p = Q.front();
Q.pop();
if(p.x == e)
{
printf("%d\n", p.step);
return 1;
}
for(int i = 0; i < tot; i++)
{
q = p;
q.x = prime[i];
if(check(p.x, q.x) && !flag[q.x])
{
q.step++;
flag[q.x] = 1;
Q.push(q);
}
}
}
return 0;
}
int main()
{
init();
int T;
scanf("%d", &T);
while(T--)
{
memset(flag, 0, sizeof(flag));
scanf("%d%d", &s, &e);
if(s == e)
{
printf("0\n");
continue;
}
int t = bfs();
if(!t) printf("impossible\n");
}
}