LeetCode Maximum Average Subarray I

原题链接在这里：https://leetcode.com/problems/maximum-average-subarray-i/description/

题目：

Given an array consisting of n integers, find the contiguous subarray of given length k that has the maximum average value. And you need to output the maximum average value.

Example 1:

Input: [1,12,-5,-6,50,3], k = 4
Output: 12.75
Explanation: Maximum average is (12-5-6+50)/4 = 51/4 = 12.75

Note:

1. 1 <= k <= n <= 30,000.
2. Elements of the given array will be in the range [-10,000, 10,000].

题解：

维护长度为k的sliding window的最大sum.

Time Complexity: O(nums.length). Space: O(1).

AC Java:

class Solution {
    public double findMaxAverage(int[] nums, int k) {
        if(nums == null || nums.length < k){
            throw new IllegalArgumentException("Illegal input array length smaller than k.");
        }
        
        long sum = 0;
        for(int i = 0; i<k; i++){
            sum += nums[i];
        }
        
        long max = sum;
        for(int i = k; i<nums.length; i++){
            sum += nums[i]-nums[i-k];
            max = Math.max(max, sum);
        }
        return max*1.0/k;
    }
}