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How did you do the backtracking?? I got only 3sec and something using back. What optimisation did you do??

Sorry, I considered that 1 is prime. But still WA... Why?

I'm using Fermat's little theorem to check the primality of a given number.
In order not to have problems with pseudoprime numbers, I also made a list with them (I took it from the Internet). So, every time I found a number that can be prime I check the list. If the list contains that number then ...

while(scanf("%d", &x) == 1)
{
if(!prime(x)) printf("%d is not prime.n", x);
else
if(!prime(reverse(x)) || x == reverse(x)) printf("%d is prime.n", x);
else printf("%d is emirp.n", x);
}

This is my (main) source code.
Why do I get P.E.?

prime(x) - tests if x is prime
reverse(x) - returns x ...

This is my source...

#include <stdio.h>

#define min(a, b) ((a) < (b) ? (a) : (b))
#define max(a, b) ((a) > (b) ? (a) : (b))

// intersection with a verical line

int vli(long long xs, long long ys, long long xe, long long ye, long long x, long long y1, long long y2)
{
if(min(xs, xe) > x || max(xs ...