USACO Prime Palindromes

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Prime Palindromes

The number 151 is a prime palindrome because it is both a prime number and a palindrome (it is the same number when read forward as backward). Write a program that finds all prime palindromes in the range of two supplied numbers a and b (5 <= a < b <= 100,000,000); both a and b are considered to be within the range .

PROGRAM NAME: pprime

INPUT FORMAT

Line 1:Two integers, a and b

SAMPLE INPUT (file pprime.in)

5 500

OUTPUT FORMAT

The list of palindromic primes in numerical order, one per line.

SAMPLE OUTPUT (file pprime.out)

5711101131151181191313353373383

HINTS (use them carefully!)

       HINT 1          HINT 2   

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Submission file Name:  

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/*ID: qhn9992PROG: pprimeLANG: C++11*/#include <iostream>#include <cstdio>#include <cstring>#include <cmath>#include <algorithm>using namespace std;bool isprime(int x){    if(x==1) return false;    int k=sqrt(x);    for(int i=2;i<=k;i++)    {        if(x%i==0) return false;    }    return true;}int a[12],A,B,flag;void dfs(int n,int x){    if(!flag) return ;    int ed=(n%2)? n/2+1:n/2;    if(x==ed)    {        if(a[0]==0||a[n-1]==0) return ;        int temp=0;        for(int i=0;i<n;i++) temp=temp*10+a[i];        if(temp<A) return ;        if(temp>B)        {            flag=0;            return ;        }        if(isprime(temp)) printf("%d\n",temp);        return ;    }    for(int i=0;i<=9;i++)    {        a[x]=a[n-1-x]=i;        if(x==0&&a[x]%2==0) continue;        dfs(n,x+1);    }}int main(){    freopen("pprime.in","r",stdin);    freopen("pprime.out","w",stdout);    scanf("%d%d",&A,&B);    int w1=floor(log(A)/log(10))+1;    int w2=floor(log(B)/log(10))+1;    for(int i=w1;i<=w2;i++)    {        flag=1;        dfs(i,0);    }    return 0;}