1918. Kth Smallest Subarray Sum

https://leetcode.com/problems/kth-smallest-subarray-sum

Description

Given an integer array nums of length n and an integer k, return the kth smallest subarray sum.

A subarray is defined as a non-empty contiguous sequence of elements in an array. A subarray sum is the sum of all elements in the subarray.

Example 1:

**Input:** nums = [2,1,3], k = 4
**Output:** 3
**Explanation:** The subarrays of [2,1,3] are:
- [2] with sum 2
- [1] with sum 1
- [3] with sum 3
- [2,1] with sum 3
- [1,3] with sum 4
- [2,1,3] with sum 6 
Ordering the sums from smallest to largest gives 1, 2, 3, 3, 4, 6. The 4th smallest is 3.

Example 2:

**Input:** nums = [3,3,5,5], k = 7
**Output:** 10
**Explanation:** The subarrays of [3,3,5,5] are:
- [3] with sum 3
- [3] with sum 3
- [5] with sum 5
- [5] with sum 5
- [3,3] with sum 6
- [3,5] with sum 8
- [5,5] with sum 10
- [3,3,5], with sum 11
- [3,5,5] with sum 13
- [3,3,5,5] with sum 16
Ordering the sums from smallest to largest gives 3, 3, 5, 5, 6, 8, 10, 11, 13, 16. The 7th smallest is 10.

Constraints:

• n == nums.length

• 1 <= n <= 2 * 104

• 1 <= nums[i] <= 5 * 104

• 1 <= k <= n * (n + 1) / 2

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