Given an integer array, nums, sort it in ascending order and return the sorted array.

You must implement the sorting algorithm yourself; do not use any built-in sorting functions. The solution must run in $O(n \log n)$ time and use the minimum possible extra space.

Constraints:

• $1 \leq$ nums.length $\leq 10^3$

• $-10^3 \leq$ nums[i] $\leq 10^3$

The problem requires sorting an array in ascending order with $O(n \log n)$ time complexity and the minimum possible space complexity, without using built-in sort functions. Considering the constraints and requirements, we can use any of the following sorting algorithms:

1. Merge sort: It guarantees $O(n \log n)$ ...