The postMerge Sort in Golang with Examples first appeared onQvault.

Merge sort is a recursive sorting algorithm and, luckily for us, it’s quite a bit faster thanbubble sort. Merge sort is adivide and conquer algorithm.

Divide

• Divide the input slice into two (equal) halves
• Recursively sort the two halves

Conquer

• Merge the two halves to form a sorted array

Full example of the merge sort algorithm

Merge sort actually has two functions involved, the recursivemergeSort function, and themerge function.

Let’s write themergeSort() function first. It’s a recursive function, which means it calls itself, and in this case, it actually calls itselftwice. The point of themergeSort function is to split the array into two roughly equal parts, call itself on those parts, then callmerge() to fit those halves back together.

funcmergeSort(items[]int)[]int{iflen(items)<2{returnitems}first:=mergeSort(items[:len(items)/2])second:=mergeSort(items[len(items)/2:])returnmerge(first,second)}<smallid="shcb-language-1"><span>Codelanguage:</span><span>Go</span><span>(</span><span>go</span><span>)</span></small>

Themerge() function is used for merging two sorted lists back into a single sorted list, its where the “magic” really happens. At the lowest level of recursion, the two “sorted” lists will each have a length of 1. Those single element lists will be merged into a sorted list of length two, and we can build of from there.

funcmerge(a[]int,b[]int)[]int{final:=[]int{}i:=0j:=0fori<len(a)&&j<len(b){ifa[i]<b[j]{final=append(final,a[i])i++}else{final=append(final,b[j])j++}}for;i<len(a);i++{final=append(final,a[i])}for;j<len(b);j++{final=append(final,b[j])}returnfinal}<smallid="shcb-language-2"><span>Codelanguage:</span><span>Go</span><span>(</span><span>go</span><span>)</span></small>

Using the algorithm in code

funcmain(){unsorted:=[]int{10,6,2,1,5,8,3,4,7,9}sorted:=mergeSort(unsortedInput)// sorted = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]}<smallid="shcb-language-3"><span>Codelanguage:</span><span>Go</span><span>(</span><span>go</span><span>)</span></small>

Why use merge sort?

Pros

• Fast. Merge sort is much faster than bubble sort, beingO(n*log(n)) instead ofO(n^2).
• Stable. Merge sort is also astable sort which means that values with duplicate keys in the original list will be in the same order in the sorted list.

Cons

• Extra memory. Most sorting algorithms can be performed using a single copy of the original array. Merge sort requires an extra array in memory to merge the sorted subarrays.
• Recursive: Merge sort requires many recursive function calls, and function calls can have significant resource overhead.

If you need a sorting algorithm to use in a production system, I recommendnot reinventing the wheel and using the built-in sort.Sort method.

Merge sort Big-O complexity

Merge sort has a complexity ofO(n*log(n)). Don’t be fooled because there aren’t an explicit number of for-loops to count in the code. In merge sort’s case, the number of recursive function calls is important.

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