Subarray Product Less Than K问题及解法

问题描述：

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

.

示例:

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.

问题分析：

利用双指针，始终寻找product小与k的子数组，统计这样的子数组个数，这里的关键是统计方法的确定。

过程详见代码：

class Solution {
public:
int n
4000
umSubarrayProductLessThanK(vector<int>& nums, int k) {
if (k <= 1) return 0;
int n = nums.size(), prod = 1, ans = 0, left = 0;
for (int i = 0; i < n; i++) {
prod *= nums[i];
while (prod >= k) prod /= nums[left++];
ans += i - left + 1;
}
return ans;
}
};