CF#315 C

#include <cstdio>
#include <iostream>
#include <algorithm>
#include <queue>
#include <cmath>
#include <cstring>
#include <stack>
#include <set>
#include <map>
#include <vector>
using namespace std;
#define INF 0x2fffffff
#define LL long long
#define MAX(a,b) ((a)>(b))?

(a):(b)
#define MIN(a,b) ((a)<(b))?(a):(b)
#define maxn 10000000
int sum1[maxn];
int is[maxn];
int sum2[maxn];
int fun(){
    int t = sqrt(maxn);
    is[0] = 1;
    is[1] = 1;
    for(int i = 0;i < t;i++){
        if(!is[i]){
            for(int j = i+i;j < maxn;j+=i){
                is[j] = 1;
            }
        }
    }
    sum1[0] = 0;
    for(int i = 1;i < maxn;i++){
        sum1[i]=!is[i] + sum1[i-1];
    }
    sum2[0] = 0;
    for(int i = 1;i < maxn;i++){
        int cnt = 0;
        int n = i;
        int tmp = 0;
        while(n){
            tmp *= 10;
            tmp += n%10;
            n /= 10;
            cnt++;
        }
        if(tmp == i){
            sum2[i] = sum2[i-1] + 1;
        }
        else{
            sum2[i] = sum2[i-1];
        }
    }
}
int main(){
    fun();
    LL p,q;
    cin >> p >> q;
    for(int i = maxn-1;i > 0;i--){
        if(q*sum1[i] <= p * sum2[i]){
            printf("%d\n",i);
            break;
        }
    }
    return 0;
}

注意回文数的求法。和比較的时候 从上開始，不能从下到上