1248. Count Number of Nice Subarrays

Given an array of integers nums and an integer k. A subarray is called nice if there are k odd numbers on it.

Return the number of nice sub-arrays.

Example 1:

Input: nums = [1,1,2,1,1], k = 3
Output: 2
Explanation: The only sub-arrays with 3 odd numbers are [1,1,2,1] and [1,2,1,1].

Example 2:

Input: nums = [2,4,6], k = 1
Output: 0
Explanation: There is no odd numbers in the array.

Example 3:

Input: nums = [2,2,2,1,2,2,1,2,2,2], k = 2
Output: 16

Constraints:

• 1 <= nums.length <= 50000

• 1 <= nums[i] <= 10^5

• 1 <= k <= nums.length

int atMost(const vector<int>& nums, int k) {
    int res = 0, i = 0, n = nums.size();
    for (int j = 0; j < n; ++j) {
        k -= nums[j] & 0x1;
        while (k < 0) {
            k += nums[i++] & 0x1;
        }
        res += j - i + 1;
    }
    return res;
}
int numberOfSubarrays(vector<int>& nums, int k) { // time: O(n); space: O(1)
    return atMost(nums, k) - atMost(nums, k - 1);
}