Bestcoder Round #51 (Div.2) Zball in Tina town

Wilson's theorem: When and only if p is a prime number: (p-1)!≡p-1 (mod p) This is a sufficient condition

This problem is done with Wilson's theorem, but don't know the theorem, it doesn't matter, can make a table,

Summary: Some problems of number theory even if you do not know a certain knowledge point, but also try to lay the table, see if you can find the law

#include <cstdio>int main () {for    (int i=1;i<=13;i++)    {        int sum=1;        for (int j=1;j<i;j++)            sum*=j;        printf ("%d\n", sum%i);    }    return 0;}

Run, you can observe, composite results are 0, prime number results are n-1, there is an exception, that is 4, it also output 2;

Title Link: http://acm.hdu.edu.cn/showproblem.php?pid=5391

#include <cstdio> #include <cmath>int main () {    int t;    scanf ("%d", &t);    while (t--)    {        int n;        scanf ("%d", &n);        if (n==4) {printf ("2\n"); continue;}        if (n==1) {printf ("1\n"); continue;}        int flag=1;        int M=floor (sqrt (n*1.0) +0.5);//Note that this long expression cannot be placed in the for-judging condition and will time out for        (int i=2;i<=m;i++)        {            if (n%i==0)            {                flag=0;break;            }        }        if (flag) printf ("%d\n", n-1);        else printf ("0\n");    }    return 0;}

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Bestcoder Round #51 (Div.2) Zball in Tina town