713. Subarray Product Less Than K


Problem Description

Given an array of integers nums and an integer k, the task is to find out how many contiguous subarrays (a subarray is a sequence of adjacent elements in the array) have a product of all its elements that is less than k. The integers in the array can be any integer and the order of the array must not be changed. The output should be the count of such subarrays.

Intuition

To arrive at the solution, we can think about a sliding window approach. We keep expanding our window by adding elements from the right and multiplying them until the product of all elements in the window is less than k. When the product becomes equal or greater than k, we'll shrink the window from the left until the product is less than k again.

As we slide the window to the right across the array, we can continuously make the window as large as possible such that its product is less than k. For each position i, if the product of the window s of elements to the left (up to j) is less than k, then adding the element at i can form i - j + 1 new subarrays with products less than k (each subarray ending at i and starting at any of the elements between i and j, inclusive).

The intuition behind while calculating i - j + 1, is that for each new element added to the window (when i is increased), we add all the possible subarrays that end with that element. For example, if our window includes elements nums[3], nums[4], nums[5], and we now include nums[6], then we can form the subarrays [nums[6]], [nums[5], nums[6]], [nums[4], nums[5], nums[6]], and [nums[3], nums[4], nums[5], nums[6]].

By using this approach, we can avoid recalculating the product of subarrays that have already been considered, therefore optimizing the entire process.

Solution Approach

The solution approach uses a sliding window technique to find contiguous subarrays that meet the product requirement. Here's how it breaks down:

  1. Initialization: We start with initializing three variables - ans, s, and j.

    • ans will hold the final count of the number of subarrays whose product is less than k and is initialized to 0.
    • s is used to maintain the product of the elements within the current window and is initialized to 1.
    • j represents the start of the sliding window, and it's initialized to 0.
  2. Traversing the Array: We traverse the array using variable i, which acts as the end of the sliding window. For each element, we do the following:

    • Multiply s by the current element v (the product of the window).
  3. Adjusting the Window: If the product s becomes greater than or equal to k, we need to shrink our window from the left. We do this in a while loop, which continues until the product s is smaller than k:

    • Divide s by the value at the start of the window, nums[j], to remove it from the product.
    • Move the start of the window to the right by incrementing j.
  4. Counting Subarrays: After adjusting the window such that the product s is less than k, we count the number of new subarrays that can be formed with the current end-point i. This is done by calculating i - j + 1:

    • The reason behind i - j + 1 is that each time the right end of the window moves (when i increases), all possible subarrays that end with the new element must be counted. These subarrays are all unique for this particular position of i.
  5. Accumulating the Answer: The number calculated from the step above is added to ans, accumulating the count of all valid subarrays that meet the condition.

  6. Returning the Result: Finally, after the loop has traversed the whole array, the accumulated value in ans represents the total number of contiguous subarrays where the product of all the elements is strictly less than k, and we return this value.

This approach ensures that each element is added and removed from the product at most once, leading to a time complexity of O(N), where N is the number of elements in the array. The sliding window efficiently keeps track of the product of elements within it while keeping the product under k.

Example Walkthrough

Let's walkthrough a small example to illustrate the solution approach.

Suppose we have nums = [10, 5, 2, 6] and k = 100.

  1. Initialization: We start with ans = 0, s = 1, and j = 0.

  2. Traversing the Array:

    • i = 0: We have the element 10 and s = s * 10 = 10 which is less than k. No need to adjust the window.
      • Counting Subarrays: i - j + 1 gives us 1 - 0 + 1 = 2, but the only valid subarray is [10]. So, ans = ans + 1 = 1.
    • i = 1: We have the element 5 and s = s * 5 = 50 which is still less than k.
      • Counting Subarrays: i - j + 1 gives us 2 - 0 + 1 = 3. The new valid subarrays are [5], [10, 5]. So, ans = ans + 2 = 3.
    • i = 2: We have the element 2 and s = s * 2 = 100 which is not less than k. Time to adjust the window.
      • Adjusting the Window: Since s >= k, we divide s by nums[j]. So, s = s / 10 = 10, and j = j + 1 = 1.
      • We multiply s by 2 again as our window had shrunk. Now s = 20.
      • Counting Subarrays: i - j + 1 gives us 3 - 1 + 1 = 3. The new valid subarrays are [2], [5, 2]. So ans = ans + 2 = 5.
    • i = 3: We have the element 6 and s = s * 6 = 120 which is not less than k.
      • Adjusting the Window: We continue to shrink the window. s = s / 5 = 24, and j = j + 1 = 2.
      • Counting Subarrays: i - j + 1 gives us 4 - 2 + 1 = 3. The new valid subarrays are [6], [2, 6]. So, ans = ans + 2 = 7.
  3. Returning the Result: After traversing the whole array, ans = 7, which is the count of contiguous subarrays with a product less than 100.

So, by following the algorithm's steps, we can determine that there are seven contiguous subarrays within nums that have a product less than k = 100.

Python Solution

1from typing import List
2
3class Solution:
4    def numSubarrayProductLessThanK(self, nums: List[int], k: int) -> int:
5        # Initialize count of subarrays, product of elements, and the start index
6        count_subarrays = 0
7        product = 1
8        start_index = 0
9      
10        # Iterate over the list using index and value
11        for end_index, value in enumerate(nums):
12            product *= value  # Update the product with the current value
13          
14            # When the product is equal or greater than k, divide it by the 
15            # starting value and increment the start_index to reduce the window size
16            while start_index <= end_index and product >= k:
17                product //= nums[start_index]
18                start_index += 1
19          
20            # Calculate the new subarrays with the current element
21            # Here, (end_index - start_index + 1) gives the count of subarrays ending with nums[end_index]
22            count_subarrays += end_index - start_index + 1
23      
24        # The final result is the count of subarrays satisfying the condition
25        return count_subarrays
26

Java Solution

1class Solution {
2    public int numSubarrayProductLessThanK(int[] nums, int k) {
3        int count = 0; // Initialize the count of subarrays
4        // Initialize 'start' and 'end' as the current window's start and end indices,
5        // and 'product' as the product of the elements within the window.
6        for (int start = 0, end = 0, product = 1; end < nums.length; end++) {
7            // Multiply the product by the current element.
8            product *= nums[end];
9            // While the product is not less than 'k' and 'start' is not beyond 'end'...
10            while (start <= end && product >= k) {
11                // ...divide the product by the 'start' element and move the start forward.
12                product /= nums[start++];
13            }
14            // Add to count the number of subarray combinations with the current 'end'.
15            // This is found by subtracting 'start' from 'end' and adding 1.
16            count += end - start + 1;
17        }
18        // Return the total count of subarrays where the product is less than 'k'.
19        return count;
20    }
21}
22

C++ Solution

1class Solution {
2public:
3    // Function to count the number of continuous subarrays 
4    // where the product of all elements in the subarray is less than k
5    int numSubarrayProductLessThanK(vector<int>& nums, int k) {
6        // Initialize the answer to 0
7        int count = 0;
8      
9        // Initialize two pointers defining the sliding window
10        // i is the right pointer, j is the left pointer, 
11        // and product keeps the product of all elements within the window
12        for (int right = 0, left = 0, product = 1; right < nums.size(); ++right) {
13            // Update the product by multiplying the rightmost element
14            product *= nums[right];
15          
16            // While the product is equal to or larger than k, 
17            // increment the left pointer to reduce the product 
18            // and contract the window size from the left
19            while (left <= right && product >= k) {
20                product /= nums[left++];
21            }
22          
23            // The number of subarrays with a product less than k 
24            // that end with nums[right] is equal to the size of the current window
25            count += right - left + 1;
26        }
27      
28        // Return the total count
29        return count;
30    }
31};
32

Typescript Solution

1function numSubarrayProductLessThanK(nums: number[], k: number): number {
2    let count = 0; // Initialize the count of subarrays
3    let product = 1; // Initialize the product of the current subarray
4    let left = 0; // Left index of the sliding window
5
6    // Process all numbers in the input array
7    for (let right = 0; right < nums.length; ++right) {
8        // Accumulate the product of the elements within the sliding window
9        product *= nums[right];
10
11        // If the product is greater than or equal to k,
12        // shrink the window from the left
13        while (left <= right && product >= k) {
14            product /= nums[left++];
15        }
16
17        // Add the number of subarrays ending at 'right' that have a product less than k
18        // This step utilizes the idea that if 'right' is the end of a valid subarray,
19        // then there are 'right - left + 1' possible starting positions for subarrays
20        count += right - left + 1;
21    }
22
23    // Return the total count of subarrays
24    return count;
25}
26

Time and Space Complexity

The provided code implements a two-pointer approach to find the number of contiguous subarrays where the product of all elements in the subarray is less than k.

Time Complexity

The time complexity of the code is O(n), where n is the number of elements in the input list nums. This is because there is a single loop over the elements, and within this loop, there is a while loop. However, the inner while loop does not result in a time complexity greater than O(n) since each element is visited at most twice by the pointers — once by the outer loop and at most once by the j pointer moving forward. Every element is processed by the inner loop only once when the product exceeds k, and after it's processed, the j pointer does not go back.

Space Complexity

The space complexity of the code is O(1). This is because the space used does not grow with the size of the input nums. Instead, only a fixed number of integers (ans, s, j, i, v) are used to keep the state of the algorithm, which occupies constant space.


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