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How do you merge k sorted lists?

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01

Understand the problem

Explain merging k sorted lists.

heaplinked-list
02

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Sketch your approach before reading the solution — that's what interviews test.

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03

Study the solution

Put the head of each list into a min-heap, repeatedly pop the smallest and push its successor — O(N log k) for N total elements. Alternatively merge lists pairwise (divide and conquer) for the same complexity without a heap.

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04

Read the code

Min-heap of list heads
Run Playground
import heapq

class ListNode:
    def __init__(self, val, nxt=None):
        self.val, self.next = val, nxt

def merge_k_lists(lists):
    heap = []
    for i, node in enumerate(lists):
        if node:
            heapq.heappush(heap, (node.val, i, node))   # i breaks ties
    dummy = tail = ListNode(0)
    while heap:
        val, i, node = heapq.heappop(heap)
        tail.next = node
        tail = node
        if node.next:
            heapq.heappush(heap, (node.next.val, i, node.next))
    return dummy.next


# --- demo ---
def build(vals):
    head = None
    for v in reversed(vals): head = ListNode(v, head)
    return head
def to_list(head):
    out = []
    while head: out.append(head.val); head = head.next
    return out

merged = merge_k_lists([build([1, 4, 5]), build([1, 3, 4]), build([2, 6])])
print(to_list(merged))   # [1, 1, 2, 3, 4, 4, 5, 6]
05

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