Linked Lists Combine K Sorted

hardcodingAlgorithmsLinked ListsHeap~40 min

Asked at Meta, Google, Stripe

Overview

You are given an array of k sorted linked lists. Merge all the linked lists into one sorted linked list and return the head.

Each linked list is a chain of nodes: { val, next }.

Constraints

0 <= k <= 10,000
0 <= total nodes across all lists <= 10,000
Each list is sorted in ascending order.
Node values are integers in range [-10,000, 10,000].

Examples

// lists: [1->4->5, 1->3->4, 2->6]
mergeKLists([
  { val: 1, next: { val: 4, next: { val: 5, next: null } } },
  { val: 1, next: { val: 3, next: { val: 4, next: null } } },
  { val: 2, next: { val: 6, next: null } },
]);
// => 1->1->2->3->4->4->5->6

mergeKLists([]);
// => null

mergeKLists([null]);
// => null

Solution

Reveal solution

function mergeKLists(lists) {
  if (!lists.length) return null;

  function mergeTwoLists(a, b) {
    const dummy = { val: 0, next: null };
    let curr = dummy;
    while (a && b) {
      if (a.val <= b.val) { curr.next = a; a = a.next; }
      else { curr.next = b; b = b.next; }
      curr = curr.next;
    }
    curr.next = a || b;
    return dummy.next;
  }

  while (lists.length > 1) {
    const merged = [];
    for (let i = 0; i < lists.length; i += 2) {
      const a = lists[i];
      const b = i + 1 < lists.length ? lists[i + 1] : null;
      merged.push(mergeTwoLists(a, b));
    }
    lists = merged;
  }

  return lists[0];
}

Divide and conquer: pairwise merge until one list remains. O(N log k) time.

Resources

linked-lists-combine-k-sorted.js

Linked Lists Combine K Sorted

hardcodingAlgorithmsLinked Lists

Overview

You are given an array of k sorted linked lists. Merge all the linked lists into one sorted linked list and return the head.

Each linked list is a chain of nodes: { val, next }.

Constraints

0 <= k <= 10,000
0 <= total nodes across all lists <= 10,000
Each list is sorted in ascending order.
Node values are integers in range [-10,000, 10,000].

Examples

// lists: [1->4->5, 1->3->4, 2->6]
mergeKLists([
  { val: 1, next: { val: 4, next: { val: 5, next: null } } },
  { val: 1, next: { val: 3, next: { val: 4, next: null } } },
  { val: 2, next: { val: 6, next: null } },
]);
// => 1->1->2->3->4->4->5->6

mergeKLists([]);
// => null

mergeKLists([null]);
// => null

Solution

Reveal solution

function mergeKLists(lists) {
  if (!lists.length) return null;

  function mergeTwoLists(a, b) {
    const dummy = { val: 0, next: null };
    let curr = dummy;
    while (a && b) {
      if (a.val <= b.val) { curr.next = a; a = a.next; }
      else { curr.next = b; b = b.next; }
      curr = curr.next;
    }
    curr.next = a || b;
    return dummy.next;
  }

  while (lists.length > 1) {
    const merged = [];
    for (let i = 0; i < lists.length; i += 2) {
      const a = lists[i];
      const b = i + 1 < lists.length ? lists[i + 1] : null;
      merged.push(mergeTwoLists(a, b));
    }
    lists = merged;
  }

  return lists[0];
}

Divide and conquer: pairwise merge until one list remains. O(N log k) time.

Resources

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