713. Subarray Product Less Than K

713。Subarray Product Less Than K

Given an array of integers nums and an integer k, return the number of contiguous subarrays where the product of all the elements in the subarray is strictly less than k.

Example 1:

Input: nums = [10,5,2,6], k = 100
Output: 8
Explanation: The 8 subarrays that have product less than 100 are:
[10], [5], [2], [6], [10, 5], [5, 2], [2, 6], [5, 2, 6]
Note that [10, 5, 2] is not included as the product of 100 is not strictly less than k.

Example 2:

Input: nums = [1,2,3], k = 0
Output: 0

Constraints:

• 1 <= nums.length <= 3 * 104

• 1 <= nums[i] <= 1000

• 0 <= k <= 106

My Solutions:

class Solution {
    public int numSubarrayProductLessThanK(int[] nums, int k) {
        if (k <= 1) return 0;
        int prod = 1, ans = 0, l = 0;
        for (int r = 0; r < nums.length; r++) {
            prod *= nums[r];
            // 如果结果超过k，把window的左边左移
            while (prod >= k) {
                prod  /= nums[l];
                l++;
            }
            // 这一步关键。可以组成的subarray数量是r - l + 1
            // e.g. l=0,r=0, ans+=1, [10]
            // e.g. l=0,r=1, ans+=2, [10,5],[5]
            // e.g. l=1,r=2, ans+=2, [5,2],[2]
            // e.g. l=1,r=3, ans+=3, [5,2,6],[2,6],[6] 
            ans += r - l + 1;
        }

        return ans;
    }
}