Over a million developers have joined DZone.

Recursive and Iterative Merge Sort Implementations

· Java Zone

Easily build powerful user management, authentication, and authorization into your web and mobile applications. Download this Forrester report on the new landscape of Customer Identity and Access Management, brought to you in partnership with Stormpath.

Curator's Note: In response to Stoimen Popov's Algorithm of the Week Post: Merge Sort, Chaker Nakhli pointed out that Stoimen only presented a recursive version of the merge sort algorithm. In this post, Chaker presents an iterative approach written in C#, but it can be easily converted to Java or any other language...

I find merge sort elegant and easy to implement and to understand for both iterative and recursive approaches. In this post I’ll share a quick (and probably dirty) iterative and recursive implementations of merge sort. Both versions share exactly the same merge operation. The implementation takes less than 30 lines of C#.

Recursive Merge Sort

public static T[] Recursive(T[] array, IComparer<T> comparer)
     Recursive(array, 0, array.Length, comparer);
     return array;
 private static void Recursive(T[] array, int start, int end, IComparer<T> comparer)
     if (end - start <= 1) return;
     int middle = start + (end - start) / 2;
     Recursive(array, start, middle, comparer);
     Recursive(array, middle, end, comparer);
     Merge(array, start, middle, end, comparer);

Iterative Merge Sort

public static T[] Iterative(T[] array, IComparer<T> comparer)
    for (int i = 1; i <= array.Length / 2 + 1; i *= 2)
        for (int j = i; j < array.Length; j += 2 * i)
            Merge(array, j - i, j, Math.Min(j + i, array.Length), comparer);
    return array;

Merge Function

The merge method below is used for both methods: recursive and iterative. It merges the two provided sub-arrays T[start, middle) and T[middle, end). The result of the merge cannot stored in the input array, it needs to be stored in a separate temporary array. This takes (end-start) memory space and will have a worst case space complexity O(n) where n is the size of the input array.

private static void Merge(T[] array, int start, int middle, int end, IComparer<T> comparer)
    T[] merge = new T[end-start];
    int l = 0, r = 0, i = 0;
    while (l < middle – start && r < end – middle)
        merge[i++] = comparer.Compare(array[start + l], array[middle + r]) < 0
            ? array[start + l++]
            : array[middle + r++];
    while (r < end – middle) merge[i++] = array[middle + r++];
    while (l < middle – start) merge[i++] = array[start + l++];
    Array.Copy(merge, 0, array, start, merge.Length);


As opposed to other in-place sorting algorithms, merge sort needs O(n) space to perform the merging step. On the other hand, it is a stable sort and it can be easily modified to implement external sorting for big data sets that do not fit in RAM.

The Java Zone is brought to you by Stormpath—a complete, pre-built User Management API. Want to learn how to use JWTs to protect microservices from CSRF and more? Check out this on-demand webinar with our Java Developer Evangelist, Micah Silverman.


Published at DZone with permission of Chaker Nakhli. See the original article here.

Opinions expressed by DZone contributors are their own.

The best of DZone straight to your inbox.

Please provide a valid email address.

Thanks for subscribing!

Awesome! Check your inbox to verify your email so you can start receiving the latest in tech news and resources.

{{ parent.title || parent.header.title}}

{{ parent.tldr }}

{{ parent.urlSource.name }}