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13. Reverse Linked List

easyAsked at Nubank

Reverse a singly linked list in-place; Nubank treats this as a pointer-discipline screen before harder ledger-traversal questions.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given the head of a singly linked list, reverse the list and return the new head. Solve it both iteratively and (optionally) recursively.

Constraints

  • Number of nodes is in [0, 5000]
  • -5000 <= Node.val <= 5000

Examples

Example 1

Input
head = [1,2,3,4,5]
Output
[5,4,3,2,1]

Example 2

Input
head = []
Output
[]

Approaches

1. Stack copy

Push all node values onto a stack, then pop into a new list.

Time
O(n)
Space
O(n)
function reverseList(head){ const s=[]; for(let c=head;c;c=c.next) s.push(c.val); let d={next:null},t=d; while(s.length){ t.next={val:s.pop(),next:null}; t=t.next;} return d.next; }

Tradeoff:

2. Iterative pointer flip

Walk the list once, flipping each node's next pointer to its prev. O(1) extra space.

Time
O(n)
Space
O(1)
function reverseList(head) {
  let prev = null, curr = head;
  while (curr) {
    const next = curr.next;
    curr.next = prev;
    prev = curr;
    curr = next;
  }
  return prev;
}

Tradeoff:

Nubank-specific tips

Nubank expects the in-place iterative version and will push on edge cases (empty, single-node); say out loud what each pointer means.

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Output

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