uva 10831 - Gerg's Cake(勒让德记号)

题目链接：uva 10831 - Gerg's Cake

题目大意：给定a和p，p为素数，问说是否存在x，使得x2≡a%p

解题思路：勒让德记号，判断ap−12≡1%p

#include <cstdio>#include <cstring>#include <algorithm>using namespace std;typedef long long ll;ll pow_mod (ll a, ll n, ll mod) {    ll ans = 1;    while (n) {        if (n&1)            ans = ans * a % mod;        a = a * a % mod;        n /= 2;    }    return ans;}int legendre (ll a, ll p) {    a %= p;    if (a == 0)        return 0;    if (pow_mod(a, (p-1)/2, p) == 1)        return 1;    else        return -1;}int main () {    ll a, p;    while (scanf("%lld%lld", &a, &p) == 2 && a != -1 && p != -1) {        if (legendre(a, p) < 0)            printf("No\n");        else            printf("Yes\n");    }    return 0;}