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Linked List

Reverse Linked ListEasy

Reverse a singly linked list and return the new head.

Examples

Approach

Walk the list once, and at each node, flip its `next` pointer to point backward instead of forward — but save the original `next` first, or you'll lose the rest of the list. Track the previous node as you go; by the end, what used to be the last node is now the head. A recursive version does the same rewiring, just using the call stack to hold position instead of a loop variable.

Complexity — best & worst case

Code

// Reverse a singly linked list and return the new head.

class ListNode {
  constructor(value, next = null) {
    this.value = value;
    this.next = next;
  }
}

function buildList(values) {
  const head = new ListNode(values[0]);
  let current = head;
  for (let i = 1; i < values.length; i++) {
    current.next = new ListNode(values[i]);
    current = current.next;
  }
  return head;
}

function toArray(head) {
  const out = [];
  let current = head;
  while (current) {
    out.push(current.value);
    current = current.next;
  }
  return out;
}

// --- Iterative: rewire one pointer at a time, O(1) extra space ---
function reverseListIterative(head) {
  let prev = null;
  let current = head;

  while (current) {
    const next = current.next; // save before we overwrite it
    current.next = prev;       // flip the pointer backward
    prev = current;
    current = next;
  }
  return prev; // prev is the new head
}

// --- Recursive: same idea, but the call stack tracks position instead
// of a loop variable. Uses O(n) space for the recursion. ---
function reverseListRecursive(head) {
  if (head === null || head.next === null) return head;

  const newHead = reverseListRecursive(head.next);
  head.next.next = head; // make the next node point back to this one
  head.next = null;
  return newHead;
}

// --- Example Usage ---
const list = buildList([1, 2, 3, 4, 5]);
console.log("original:", toArray(list));                    // [1,2,3,4,5]
console.log("reversed (iterative):", toArray(reverseListIterative(buildList([1, 2, 3, 4, 5])))); // [5,4,3,2,1]
console.log("reversed (recursive):", toArray(reverseListRecursive(buildList([1, 2, 3, 4, 5])))); // [5,4,3,2,1]