[LeetCode] 1343. Number of Sub-arrays of Size K and Average Greater than or Equal to Threshold

Given an array of integers arr and two integers k and threshold, return the number of sub-arrays of size k and average greater than or equal to threshold.

Example 1:
Input: arr = [2,2,2,2,5,5,5,8], k = 3, threshold = 4
Output: 3
Explanation: Sub-arrays [2,5,5],[5,5,5] and [5,5,8] have averages 4, 5 and 6 respectively. All other sub-arrays of size 3 have averages less than 4 (the threshold).

Example 2:
Input: arr = [11,13,17,23,29,31,7,5,2,3], k = 3, threshold = 5
Output: 6
Explanation: The first 6 sub-arrays of size 3 have averages greater than 5. Note that averages are not integers.

Constraints:
1 <= arr.length <= 105
1 <= arr[i] <= 104
1 <= k <= arr.length
0 <= threshold <= 104

大小为 K 且平均值大于等于阈值的子数组数目。

给你一个整数数组 arr 和两个整数 k 和 threshold 。

请你返回长度为 k 且平均值大于等于 threshold 的子数组数目。

思路

这道题是一道窗口尺寸固定的滑动窗口题。注意尽量不要在中间过程求平均值因为会涉及到精度问题。在过程中我们可以只求数字的 sum，不求平均值，到最后再计算平均值。

复杂度

时间O(n)
空间O(1)

代码

Java实现

class Solution {
    public int numOfSubarrays(int[] arr, int k, int threshold) {
        int n = arr.length;
        int sum = 0;
        for (int i = 0; i < k; i++) {
            sum += arr[i];
        }

        int res = 0;
        if (sum >= threshold * k) {
            res++;
        }
        for (int i = k; i < n; i++) {
            sum += arr[i];
            sum -= arr[i - k];
            if (sum >= threshold * k) {
                res++;
            }
        }
        return res;
    }
}