In-Place Linked List Reversal Pattern

When to use In-Place Linked List Reversal

• You need to reverse a singly linked list or a contiguous sublist of it.
• You need to reverse every k consecutive nodes (or alternate groups of k nodes).
• The problem disallows allocating a new list or recursive stack memory.
• You need to compare the original list against its reverse (palindrome check) in O(1) extra space.
• You need to rotate, reorder, or partition a list using reversal as a primitive.

Template

function reverseList(head) {
  let previous = null;
  let current = head;
  while (current !== null) {
    const next = current.next;
    current.next = previous;
    previous = current;
    current = next;
  }
  return previous;
}

Common mistakes

• Forgetting to capture `current.next` in a temporary variable before rewriting `current.next`, which loses the rest of the list.
• Returning `current` (which is null at the end) instead of `previous` as the new head.
• On sublist reversal, forgetting to stitch the original predecessor's `next` to the new sublist head, which orphans the prefix.
• Off-by-one errors when counting k nodes for k-group reversal — verifying that a full group exists before reversing it.
• Using a dummy head only sometimes and then forgetting to skip it when returning, which exposes the dummy to the caller.