RosettaCodeData/Task/Sorting-algorithms-Merge-sort/00DESCRIPTION

{{Sorting Algorithm}}
[[Category:Recursion]]

The &nbsp; '''merge sort''' &nbsp; is a recursive sort of order &nbsp; <big> n*log(n). </big>

It is notable for having a worst case and average complexity of &nbsp; <big> ''O(n*log(n))'', </big> &nbsp; and a best case complexity of &nbsp; <big> ''O(n)'' &nbsp; </big> (for pre-sorted input).

The basic idea is to split the collection into smaller groups by halving it until the groups only have one element or no elements &nbsp; (which are both entirely sorted groups).

Then merge the groups back together so that their elements are in order.

This is how the algorithm gets its &nbsp; ''divide and conquer'' &nbsp; description.


;Task:
Write a function to sort a collection of integers using the merge sort.


The merge sort algorithm comes in two parts:
    a sort function     and
    a merge function

The functions in pseudocode look like this:
 '''function''' ''mergesort''(m)
    '''var''' list left, right, result
    '''if''' length(m) ≤ 1
        '''return''' m
    '''else'''
        '''var''' middle = length(m) / 2
        '''for each''' x '''in''' m '''up to''' middle - 1
            '''add''' x '''to''' left
        '''for each''' x '''in''' m '''at and after''' middle
            '''add''' x '''to''' right
        left = mergesort(left)
        right = mergesort(right)
        '''if''' last(left) ≤ first(right)
           '''append''' right '''to''' left
           '''return''' left
        result = merge(left, right)
        '''return''' result

 '''function''' ''merge''(left,right)
    '''var''' list result
    '''while''' length(left) > 0 and length(right) > 0
        '''if''' first(left) ≤ first(right)
            '''append''' first(left) '''to''' result
            left = rest(left)
        '''else'''
            '''append''' first(right) '''to''' result
            right = rest(right)
    '''if''' length(left) > 0
        '''append''' rest(left) '''to''' result
    '''if''' length(right) > 0
        '''append''' rest(right) '''to''' result
    '''return''' result


;See also:
* &nbsp; the Wikipedia entry: &nbsp; [[wp:Merge_sort| merge sort]]


Note: &nbsp; better performance can be expected if, rather than recursing until &nbsp; <big> length(m) ≤ 1, </big> &nbsp; an insertion sort is used for &nbsp; <big> length(m) </big> &nbsp; smaller than some threshold larger than &nbsp; '''1'''. &nbsp; However, this complicates the example code, so it is not shown here.
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