How to Fix Merge Sort Errors

In this tutorial, you'll learn about How to Fix Merge Sort Errors. We cover key concepts, practical examples, and best practices.

Fix merge sort errors when merge step creates many temporary arrays or recursion doesn't handle odd lengths.

Quick Fix

Wrong

def ms(a):
    if len(a)<=1: return a
    m=len(a)//2
    l=ms(a[:m]); r=ms(a[m:])
    return l+r  # no actual merging!

No merge step. Just concatenates halves.

Right

def ms(a):
    if len(a)<=1: return a
    m=len(a)//2
    l=ms(a[:m]); r=ms(a[m:])
    i=j=0; res=[]
    while i<len(l) and j<len(r):
        if l[i]<=r[j]: res.append(l[i]); i+=1
        else: res.append(r[j]); j+=1
    return res+l[i:]+r[j:]

[38,27,43,3,9,82,10] -> [3,9,10,27,38,43,82]. O(n log n).

Prevention

Merge step: compare front of each half, pick smaller. Append remaining from non-exhausted half.

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