首先，是从一个数组中选k个，就是滑动窗口的解法，而选一个最短的数组，我原有的方法是遍历被卡了，采用了大佬的方法二分，这道题还有一个关键点，必须排序，不排序过不了，方差公式是dx=ex2-ex的平方

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

bool checkWindow(vector<int> g, int end, int k, long long t) {
    if (end + 1 < k) return false;
    sort(g.begin(), g.begin() + end + 1);

    long long sum = 0;
    long long sum_sq7 = 0;
    for (int i = 0; i < k; i++) {
        sum += g[i];
        sum_sq += (long long)g[i] * g[i];
    }
    if (sum_sq * k - sum * sum < (long long)k * k * t) {
        return true;
    }
    for (int i = k; i <= end; i++) {
        sum += g[i] - g[i - k];
        sum_sq += (long long)g[i] * g[i] - (long long)g[i - k] * g[i - k];

        if (sum_sq * k - sum * sum < (long long)k * k * t) {
            return true;
        }
    }

    return false;
}

int main() {
    int n, k, t;
    cin >> n >> k >> t;
    vector<int> g(n);

    for (int i = 0; i < n; i++) {
        cin >> g[i];
    }

    int left = k - 1;
    int right = n - 1;
    int result = -1;

    while (left <= right) {
        int mid = left + (right - left) / 2;
        if (checkWindow(g, mid, k, t)) {
            result = mid;
            right = mid - 1;
        } else {
            left = mid + 1;
        }
    }

    cout << (result != -1 ? result + 1 : -1) << endl;
    return 0;
}