Count Number of Nice Subarrays

1. Define the problem

Given an array of integers nums and an integer k, a subarray is called nice if it contains exactly k odd numbers . Return the number of nice subarrays. Count subarrays with at most k odd numbers, subtract the count with at most k minus one, and the difference is exactly k.

Constraints

1 ≤ nums.length ≤ 5 × 10⁴
1 ≤ nums_i ≤ 10⁵
0 ≤ k ≤ nums.length

Example

Inputnums = [1, 1, 2, 1, 1], k = 3

Output2

Explanation The two nice subarrays are [1,1,2,1] and [1,2,1,1], each containing exactly 3 odd numbers.

2. Visualize the solution

Count-at-most(k) minus count-at-most(k-1)

Statusgrow

Window [1,1,2,1] has 3 odd numbers <= 3; add 4 to the running count.

What happens in this step

atMost(3) — right=3, window [1,1,2,1] (left=0)
  odd values in window: 1, 1, 1 → oddCount = 3
  3 <= 3 → within limit, no shrinking
  count += (right - left + 1) = (3 - 0 + 1) = 4

Step 1 of 4

Steps to visualize

Run a variable window that counts subarrays with at most a target number of odd values.
Grow right and increment the odd counter when the new value is odd.
While the odd counter exceeds the target, shrink from the left.
At each right position, add the current window size to the count.
Call this helper for k and for k minus one, then subtract.

3. Walk through the code

Walk through the code

Same walkthrough, now with the code. Press Next to move one step and watch which lines run.

Count-at-most(k) minus count-at-most(k-1)

Statusgrow

Window [1,1,2,1] has 3 odd numbers <= 3; add 4 to the running count.

What happens in this step

atMost(3) — right=3, window [1,1,2,1] (left=0)
  odd values in window: 1, 1, 1 → oddCount = 3
  3 <= 3 → within limit, no shrinking
  count += (right - left + 1) = (3 - 0 + 1) = 4

Step 1 of 4

4. Solution

Solution

solution.tsTypeScript

function numberOfSubarrays(nums, k) {
  function atMost(limit) {
    if (limit < 0) return 0;
    let left = 0;
    let oddCount = 0;
    let count = 0;

    for (let right = 0; right < nums.length; right++) {
      if (nums[right] % 2 !== 0) oddCount++;

      while (oddCount > limit) {
        if (nums[left] % 2 !== 0) oddCount--;
        left++;
      }

      count += right - left + 1;
    }

    return count;
  }

  return atMost(k) - atMost(k - 1);
}

Time: O(n)
Space: O(1)

5. Test cases

Test cases

Input	Expected	Covers
`nums = [1, 1, 2, 1, 1], k = 3`	`2`	Docstring example
`nums = [2, 4, 6], k = 0`	`6`	k = 0 counts every run of pure evens
`nums = [1, 3, 5], k = 2`	`2`	All odd numbers
`nums = [2, 4, 6], k = 1`	`0`	k larger than the total odd count
`nums = [1], k = 1`	`1`	Single odd element
`nums = [1, 1, 1], k = 1`	`3`	Every single-element window qualifies
`nums = [2, 1, 2, 1, 2], k = 2`	`4`	Mixed evens and odds with multiple valid windows

Count Number of Nice Subarrays