Introduction

In this blog post, we will explore how to solve the LeetCode problem "1508" using AI. LeetCode is a popular platform for preparing for coding interviews, and with the help of AI tools like Chatmagic, we can generate solutions quickly and efficiently - helping you pass the interviews and get the job offer without having to study for months.

Problem Statement

You are given the array nums consisting of n positive integers. You computed the sum of all non-empty continuous subarrays from the array and then sorted them in non-decreasing order, creating a new array of n (n + 1) / 2 numbers. Return the sum of the numbers from index left to index right (indexed from 1), inclusive, in the new array. Since the answer can be a huge number return it modulo 109 + 7. Example 1: Input: nums = [1,2,3,4], n = 4, left = 1, right = 5 Output: 13 Explanation: All subarray sums are 1, 3, 6, 10, 2, 5, 9, 3, 7, 4. After sorting them in non-decreasing order we have the new array [1, 2, 3, 3, 4, 5, 6, 7, 9, 10]. The sum of the numbers from index le = 1 to ri = 5 is 1 + 2 + 3 + 3 + 4 = 13. Example 2: Input: nums = [1,2,3,4], n = 4, left = 3, right = 4 Output: 6 Explanation: The given array is the same as example 1. We have the new array [1, 2, 3, 3, 4, 5, 6, 7, 9, 10]. The sum of the numbers from index le = 3 to ri = 4 is 3 + 3 = 6. Example 3: Input: nums = [1,2,3,4], n = 4, left = 1, right = 10 Output: 50 Constraints: n == nums.length 1 <= nums.length <= 1000 1 <= nums[i] <= 100 1 <= left <= right <= n (n + 1) / 2

Explanation

Here's a breakdown of the approach, complexity, and the Python code:

• High-Level Approach:

  • Generate all subarray sums.
  • Sort the subarray sums.
  • Calculate the sum of the sorted subarray sums within the specified range (left to right) modulo 10⁹ + 7.

• Complexity:

  • Runtime Complexity: O(n² log n), dominated by sorting the subarray sums (n² is the number of subarray sums).
  • Storage Complexity: O(n²) to store all subarray sums.

Code

    def rangeSum(nums, n, left, right):
    """
    Calculates the sum of sorted subarray sums within a specified range.

    Args:
        nums: The input array of positive integers.
        n: The length of the input array.
        left: The starting index of the range (1-indexed).
        right: The ending index of the range (1-indexed).

    Returns:
        The sum of the numbers from index left to index right in the sorted subarray sums array, modulo 10^9 + 7.
    """

    subarray_sums = []
    for i in range(n):
        current_sum = 0
        for j in range(i, n):
            current_sum += nums[j]
            subarray_sums.append(current_sum)

    subarray_sums.sort()

    total_sum = 0
    for i in range(left - 1, right):
        total_sum = (total_sum + subarray_sums[i]) % (10**9 + 7)

    return total_sum