问题：

Your are given an array of positive integers nums.

Count and print the number of (contiguous) subarrays where the product of all the elements in the subarray is less than k.

Example 1:

Input: nums = [10, 5, 2, 6], k = 100Output: 8Explanation: 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.

Note:

• 0 < nums.length <= 50000.
• 0 < nums[i] < 1000.
• 0 <= k < 10^6.

解决：

①  双指针 + 滑动窗口

class Solution { //21ms

    public int numSubarrayProductLessThanK(int[] nums, int k) {         if (k <= 1) return 0;         int res = 0;         int val = 1;         int left = 0;         for (int i = 0;i < nums.length;i ++){             val *= nums[i];             while(val >= k) val /= nums[left ++];             res += i - left + 1;         }         return res;     } }