1. 乘积小于 K 的子数组
给定一个正整数数组 nums和整数 k ，请找出该数组内乘积小于 k 的连续的子数组的个数。
方法一：

def numSubarrayProductLessThanK(nums, k):
    n = len(nums)
    if n == 0:
        return 0
    start = 0
    end = 0
    acc = 1
    res = 0
    while end < n:
        acc *= nums[end]
        while acc >= k and start <= end:
            acc /= nums[start]
            start += 1
        if start <= end:
            res += end - start + 1
        end += 1
    return res

方法二：

def numSubarrayProductLessThanK(nums, k):
    n = len(nums)
    res = 0
    for i in range(n):
        product = 1
        for j in range(i, n):
            product *= nums[j]
            if product < k:
                res += 1
            else:
                break
    return res

方法三：

def numSubarrayProductLessThanK(nums, k):
    n = len(nums)
    res = 0
    left = 0
    product = 1
    for right in range(n):
        product *= nums[right]
        while left <= right and product >= k:
            product /= nums[left]
            left += 1
        res += right - left + 1
    return res