— 题解传送门 —

二分

C++ 代码

#include <bits/stdc++.h>
const double eps = 1e-5;
const int maxn = 1e5 + 100;
typedef long long LL;

template <typename T>
inline void read(T &s) {
    s = 0;
    T w = 1, ch = getchar();
    while (!isdigit(ch)) { if (ch == '-') w = -1; ch = getchar(); }
    while (isdigit(ch))  { s = (s << 1) + (s << 3) + (ch ^ 48); ch = getchar(); }
    s *= w;
}

int n;
int f;
int a[maxn];
double l;
double r;
double mid;
double sum[maxn];

bool check(double avg) {
    for (int i = 1; i <= n; ++i)
        sum[i] = sum[i - 1] + a[i] - avg;

    double minv = 0;
    for (int i = 0, j = f; j <= n; ++i, ++j) {
        minv = std::min(sum[i], minv);
        if (sum[j] - minv >= 0) return true;
    }
    return false;
}

int main() {
    l = 0, r = 0;
    read(n), read(f);
    for (int i = 1; i <= n; ++i) {
        read(a[i]);
        r = std::max(r, (double)a[i]);
    }

    while (r - l > eps) {
        mid = (l + r) / 2;
        if (check(mid)) l = mid;
        else r = mid;
    }

    printf("%d\n", (int)(r * 1000));
    return 0;
}