Merge Sort

This is to demonstrate the divide and conquer algorithm for the sorting problem.

Sorting link

The sorting problem is defined as below: Given n numbers, sort them in ascending order. For example, given 5, 4, 11, 2, 10, 8, 4, the sorted result is 2, 4, 4, 5, 8, 10, 11.

Algorithm link

The divide and conquer algorithm for the sorting problem is called merge sort.

The basic idea is as follows:

1. Divide the n numbers into two subsequences of n/2 numbers each (if n = 1 then return).
2. Conquer the two subsequences recursively using the divide and conquer algorithm itself.
3. Merge the two sorted subsequences to produce the sorted result.

Example link

Input [5, 4, 11, 2, 10, 8, 4]

Divide

1. The list [5, 4, 11, 2, 10, 8, 4] is divided into sub-lists [5, 4, 11, 2] and [10, 8, 4].
2. These sub-lists are further divided:
   • [5, 4, 11, 2] becomes [5, 4] and [11, 2].
   • [10, 8, 4] becomes [10, 8] and [4].
3. This continues until each sub-list has only one element: [5], [4], [11], [2], [10], [8], [4].

Conquer

1. Merge pairs of sub-lists:
   • [5] and [4] merge to [4, 5].
   • [11] and [2] merge to [2, 11].
   • [10] and [8] merge to [8, 10].
   • [4] remains [4].
2. Merge sorted sub-arrays:
   • [4, 5] and [2, 11] merge to [2, 4, 5, 11].
   • [8, 10] and [4] merge to [4, 8, 10].
3. Final merge:
   • [2, 4, 5, 11] and [4, 8, 10] merge to [2, 4, 4, 5, 8, 10, 11].

Code link

Define two functions:

• merge_sort to sort the list (divide).
• merge to merge two sorted sub-lists (conquer).

  
def merge_sort(arr):
    if len(arr) <= 1:
        return arr
    mid = len(arr) // 2
    left = merge_sort(arr[:mid])
    right = merge_sort(arr[mid:])
    return merge(left, right)
  

  
def merge(left, right):
    result = []
    i = j = 0
    while i < len(left) and j < len(right):
        if left[i] < right[j]:
            result.append(left[i])
            i += 1
        else:
            result.append(right[j])
            j += 1
    result += left[i:]
    result += right[j:]
    return result
  

Run the code as below and yield the sorted result.

  
if __name__ == '__main__':
    arr = [5, 4, 11, 2, 10, 8, 4]
    print(merge_sort(arr))