713 - Subarray Product Less Than K

#medium

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

class Solution {
public:
    int numSubarrayProductLessThanK(vector<int>& nums, int k) {
        // 看到連續子陣列 sliding window + two pointer
        // 只有正數且越乘越大
        if ( k <= 1 ) return 0;
        int left = 0;
        int result = 0;
        int product = 1;
        for ( int right = 0; right < nums.size(); right++ ) {
            product *= nums[right];
            while ( product >= k ) {
                product /= nums[left];
                left++;
            } 
            
            result += right - left + 1; // [20, 30], k = 2,  0 - 1 + 1 = 0.
        }

        return result;
    }
};