周武王补课

题目叙述

方法：

期望DP

#include <bita/stdc++.h>
const int maxn = 100010;
double p[maxn], g[maxn], f[maxn];
int main()
{
    int n;
    scanf("%d", &n);
    for (int i = 1; i <= n; ++i) scanf("%lf", p + i);
    for (int i = 1; i <= n; ++i)
    {
        g[i] = p[i] * (g[i - 1] + 1.f);
        f[i] = p[i] * (f[i - 1] + 2.f * g[i - 1] + 1.f) + (1.f - p[i]) * f[i - 1];
    }
    printf("%.3lf\n", f[n]);
    return 0;
}