CF 568A(Primes or Palindromes?-暴力判断)

A. Primes or Palindromes?

time limit per test

memory limit per test

input

output

Rikhail Mubinchik believes that the current definition of prime numbers is obsolete as they are too complex and unpredictable. A palindromic number is another matter. It is aesthetically pleasing, and it has a number of remarkable properties. Help Rikhail to convince the scientific community in this!

prime

palindromic

π(n) — the number of primes no larger than n, rub(n) — the number of palindromic numbers no larger than n. Rikhail wants to prove that there are a lot more primes than palindromic ones.

A find the maximum n, such that π(n) ≤ A·rub(n).

Input

p, q, the numerator and denominator of the fraction that is the value of A (

, 

).

Output

"Palindromic tree is better than splay tree"

Sample test(s)

input

1 1

output

40

input

1 42

output

1

input

6 4

output

172

可以发现不可能无解，极限情况n不大

#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<algorithm>
#include<functional>
#include<iostream>
#include<cmath>
#include<cctype>
#include<ctime>
using namespace std;
#define For(i,n) for(int i=1;i<=n;i++)
#define Fork(i,k,n) for(int i=k;i<=n;i++)
#define Rep(i,n) for(int i=0;i<n;i++)
#define ForD(i,n) for(int i=n;i;i--)
#define RepD(i,n) for(int i=n;i>=0;i--)
#define Forp(x) for(int p=pre[x];p;p=next[p])
#define Forpiter(x) for(int &p=iter[x];p;p=next[p])  
#define Lson (x<<1)
#define Rson ((x<<1)+1)
#define MEM(a) memset(a,0,sizeof(a));
#define MEMI(a) memset(a,127,sizeof(a));
#define MEMi(a) memset(a,128,sizeof(a));
#define INF (2139062143)
#define F (100000007)
typedef long long ll;
ll mul(ll a,ll b){return (a*b)%F;}
ll add(ll a,ll b){return (a+b)%F;}
ll sub(ll a,ll b){return (a-b+llabs(a-b)/F*F+F)%F;}
void upd(ll &a,ll b){a=(a%F+b%F)%F;}
bool is_prime(int x)
{
  if (x==1) return 0;
  Fork(i,2,sqrt(x))
  {
    if (x%i==0) return 0;
  }
  return 1;
}
const int MAXN =10000000;
int P[MAXN],siz=0,b[MAXN]={0};
void make_prime(int n)
{
  Fork(i,2,n)
  {
    if (!b[i])
    {
      P[++siz]=i;
    }
    For(j,siz)
    {
      if (P[j]*i>n) break;
      b[P[j]*i]=1;
      if (i%P[j]==0) break;
    }
  }
}
bool is_pal(int x)
{
  char s[10];
  sprintf(s,"%d",x);
  int p=0,q=strlen(s)-1;
  while(p<q) if (s[p]!=s[q]) return 0;else ++p,--q;
  return 1;
}

bool B[MAXN]={0};
bool make_pal(int n)
{
  char s[20];
  For(i,10000)
  {
    
    sprintf(s,"%d",i);
    int m=strlen(s);
    int p=m-1;
    for(int j=m;p>-1;j++,p--) s[j]=s[p];
    
    int x;
    sscanf(s,"%d",&x);
    if (x<=n) B[x]=1;
    
    for(int j=m;j<=2*m-1;j++) s[j]=s[j+1];
    sscanf(s,"%d",&x);
    if (x<=n) B[x]=1;
    
  } 
}

int main()
{
//  freopen("A.in","r",stdin);
//  freopen(".out","w",stdout);
  int p,q;
  cin>>p>>q; 
  make_prime(MAXN-1);
  make_pal(MAXN-1);  
  int x1=0,x2=0,n=MAXN-1,ans=1,t=1;
  For(i,n)
  {
    if (i==P[t]) x1++,t++;
    if (B[i]) x2++;
    if ((ll)(x1)*q<=(ll)(x2)*p) ans=i;
  }
  cout<<ans<<endl;
  return 0;
}