How to Fix Merge K Sorted Lists Errors

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

Fix merge k sorted lists errors when comparing all k heads each iteration instead of using heap.

Quick Fix

Wrong

def merge(lists):
    vals=[]
    for l in lists:
        while l: vals.append(l.val); l=l.next
    vals.sort()
    return vals

O(N log N) where N = total nodes. Doesn't use sorted property.

Right

import heapq
def merge(lists):
    h=[(l.val,i,l) for i,l in enumerate(lists) if l]
    heapq.heapify(h)
    d=c=ListNode(0)
    while h:
        v,i,n=heapq.heappop(h)
        c.next=n; c=c.next
        if n.next: heapq.heappush(h,(n.next.val,i,n.next))
    return d.next

[[1,4,5],[1,3,4],[2,6]] -> [1,1,2,3,4,4,5,6]. O(N log k).

Prevention

Push head of each list into min-heap. Pop smallest, push its next.

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