UVA 10883 - Supermean（组合数学+数值优化）

#include <stdio.h>
#include <string.h>
#include <math.h>
#define min(a,b) ((a)<(b)?(a):(b))
const int N = 50005;
int t, n;
double a, c;

double cal(int i, double a) {
	return c + log(a) - (n - 1) * log(2);
}

int main() {
	int cas = 0;
	scanf("%d", &t);
	while (t--) {
		scanf("%d", &n);
		double sum = 0;
		c = 0;
		for (int i = 0; i < n; i++) {
			scanf("%lf", &a);
			if (a < 0) sum -= exp(cal(i, -a));
			else sum += exp(cal(i, a));
			c = c + log(n - i - 1) - log(i + 1);
		}
		printf("Case #%d: %.3lf\n", ++cas, sum);
	}
	return 0;
}