Rearrange Linked List

hardcodingAlgorithmsLinked ListsTwo Pointers~30 min

Asked at Meta, Google

Overview

Given a singly linked list L0 → L1 → … → Ln-1 → Ln, reorder it to: L0 → Ln → L1 → Ln-1 → L2 → Ln-2 → …

You must modify the list in-place — do not create new nodes, only rearrange the pointers.

Constraints

The list has 1 to 50,000 nodes.
Node values are integers.
Modify the list in place and return the head.

Examples

// 1 -> 2 -> 3 -> 4
reorderList(head);
// => 1 -> 4 -> 2 -> 3

// 1 -> 2 -> 3 -> 4 -> 5
reorderList(head);
// => 1 -> 5 -> 2 -> 4 -> 3

Solution

Reveal solution

function reorderList(head) {
  if (!head || !head.next) return head;

  // 1. Find middle
  let slow = head, fast = head;
  while (fast.next && fast.next.next) {
    slow = slow.next;
    fast = fast.next.next;
  }

  // 2. Reverse second half
  let prev = null, curr = slow.next;
  slow.next = null;
  while (curr) {
    const next = curr.next;
    curr.next = prev;
    prev = curr;
    curr = next;
  }

  // 3. Merge alternating
  let first = head, second = prev;
  while (second) {
    const tmp1 = first.next, tmp2 = second.next;
    first.next = second;
    second.next = tmp1;
    first = tmp1;
    second = tmp2;
  }

  return head;
}

Three steps: find middle with slow/fast, reverse the second half, merge by alternating nodes.

Resources

rearrange-linked-list.js

Rearrange Linked List

hardcodingAlgorithmsLinked Lists

Overview

Given a singly linked list L0 → L1 → … → Ln-1 → Ln, reorder it to: L0 → Ln → L1 → Ln-1 → L2 → Ln-2 → …

You must modify the list in-place — do not create new nodes, only rearrange the pointers.

Constraints

The list has 1 to 50,000 nodes.
Node values are integers.
Modify the list in place and return the head.

Examples

// 1 -> 2 -> 3 -> 4
reorderList(head);
// => 1 -> 4 -> 2 -> 3

// 1 -> 2 -> 3 -> 4 -> 5
reorderList(head);
// => 1 -> 5 -> 2 -> 4 -> 3

Solution

Reveal solution

function reorderList(head) {
  if (!head || !head.next) return head;

  // 1. Find middle
  let slow = head, fast = head;
  while (fast.next && fast.next.next) {
    slow = slow.next;
    fast = fast.next.next;
  }

  // 2. Reverse second half
  let prev = null, curr = slow.next;
  slow.next = null;
  while (curr) {
    const next = curr.next;
    curr.next = prev;
    prev = curr;
    curr = next;
  }

  // 3. Merge alternating
  let first = head, second = prev;
  while (second) {
    const tmp1 = first.next, tmp2 = second.next;
    first.next = second;
    second.next = tmp1;
    first = tmp1;
    second = tmp2;
  }

  return head;
}

Three steps: find middle with slow/fast, reverse the second half, merge by alternating nodes.

Resources

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