713.Subarray Product Less Than K

双指针

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

分析：双指针，让范围内积始终<k。

res+= right-left+1 这里是计算每次增加的subarray数量， 每增加一个right, 都计算新的end with right的subarray 数量

```python3
class Solution:
    def numSubarrayProductLessThanK(self, nums: List[int], k: int) -> int:
        if k <= 1:
            return 0
        left=0
        n = len(nums)
        product = 1
        res = 0
        for right in range(n):
            product *= nums[right]
            while product >= k:
                product //= nums[left]
                left += 1
            res += right-left+1
        return res

```