Merge sort is a sorting technique that is based on the divide and conquer method. It is one of the most respected algorithms, with a worst-case time complexity of O(n log n).
Merge sort divides the array into equal parts before combining them in a sorted fashion.
Merge sort keeps on dividing the list into equal halves until it can no more be divided. By definition, if it is only one element in the list, it is sorted. Then, merge sort combines the smaller sorted lists keeping the new list sorted too.
Step 1 − if it is only one element in the list it is already sorted, return. Step 2 − divide the list recursively into two halves until it can no more be divided. Step 3 − merge the smaller lists into new list in sorted order.
We shall now see the pseudocodes for merge sort functions. As our algorithms point out two main functions − divide & merge.
Merge sort works with recursion and we shall see our implementation in the same way.
procedure mergesort( var a as array ) if ( n == 1 ) return a var l1 as array = a ... a[n/2] var l2 as array = a[n/2+1] ... a[n] l1 = mergesort( l1 ) l2 = mergesort( l2 ) return merge( l1, l2 ) end procedure procedure merge( var a as array, var b as array ) var c as array while ( a and b have elements ) if ( a > b ) add b to the end of c remove b from b else add a to the end of c remove a from a end if end while while ( a has elements ) add a to the end of c remove a from a end while while ( b has elements ) add b to the end of c remove b from b end while return c end procedure