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:


Solution:

class Solution {
    public int numberOfSubarrays(int[] nums, int k) {
        // [1, 1, 2, 1, 1]
        // [1, 2, 2, 3, 4]
        int n = nums.length;
        int pre = 0;
        Map<Integer, Integer> map = new HashMap();
        map.put(0, 1);
        int res = 0;
        for (int i = 1; i <= n; i ++) {
            pre = pre + (nums[i - 1] % 2 == 1 ? 1 : 0);
            int target = pre - k;
            if (map.containsKey(target)) {
                res += map.get(target);
            }
            map.put(pre, map.getOrDefault(pre, 0) + 1);
            
        }
        return res;
    }
}