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Count Windows With All Evens

Published at24 Jul 2026
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Given an integer array and window size k, count windows where all elements are even numbers (divisible by 2).

Track the number of odd elements in the current window using a sliding counter. When the counter is 0, all elements are even, so increment the result.

O(n) time, O(1) space.

Example Input & Output

Example 1
Input
[],2
Output
0
Example 2
Input
[1,3,5],2
Output
0
Explanation

No evens.

Example 3
Input
[2],1
Output
1
Explanation

Single even.

Example 4
Input
[2,4,6,8],3
Output
2
Explanation

[2,4,6] and [4,6,8] all even.

Example 5
Input
[2,3,4],2
Output
1
Explanation

Only [2,4] is both even.

Algorithm Flow

Recommendation Algorithm Flow for Count Windows With All Evens
Recommendation Algorithm Flow for Count Windows With All Evens

Solution Approach

Count odds in first window. Slide: adjust counter as elements leave/enter. When odd count is 0, all are even.

Best Answers

python
def solution(nums, k):
    if len(nums)<k: return 0
    odd=sum(1 for i in range(k) if nums[i]%2)
    res=1 if odd==0 else 0
    for i in range(k,len(nums)):
        if nums[i-k]%2: odd-=1
        if nums[i]%2: odd+=1
        if odd==0: res+=1
    return res