乘积最大（蓝桥杯18J）

#include<iostream>
#include<algorithm>
using namespace std;
typedef long long LL;
const int N = 100010, mod = 1000000009;
int a[N];
int main() {
	int n, k;
	cin >> n >> k;
	for (int i = 0; i < n;i++) {
		cin >> a[i];
	}
	sort(a,a+n);
	LL res = 1;
	int l = 0, r = n - 1;
	int sign = 1;
	if (k%2) {
		res = a[r];
		r--;
		k--;
		if (res < 0) {
			sign = -1;
		}
	}
	while (k) {
		LL x = LL(a[l] * a[l + 1]), y = LL(a[r]*a[r-1]);
		if (x*sign >y *sign) {
			res = x % mod * res % mod;
			l += 2;
		}
		else {
			res = y % mod * res % mod;
			r -= 2;
		}
		k -= 2;
	}
	cout << res;
	return 0;
}