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0713 - Subarray Product Less Than K (Medium)

https://leetcode.com/problems/subarray-product-less-than-k/

Problem Statement​

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 thank.

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 * 10^4
  • 1 <= nums[i] <= 1000
  • 0 <= k <= 10^6

Approach 1: Sliding Window​

Written by @wkw
class Solution {
public:
    int numSubarrayProductLessThanK(vector<int>& nums, int k) {
        // edge case
        if (k <= 1) return 0;
        int n = nums.size(), p = 1, ans = 0, i = 0;
        // standard sliding window
        for (int j = 0; j < n; j++){
            // step 1: expand the window
            p *= nums[j];
            // step 2: shrink the window if applicable
            while (p >= k) p /= nums[i++];
            // step 3: add the window size to answer
            ans += j - i + 1;
        }
        return ans;
    }
};