Linked list reversal

O(n)

Walk the list once, and at each node flip its .next pointer to point backward instead of forward, carrying prev/curr/next along so nothing gets lost.

Signals

reverse a linked list (whole list or between positions)reverse in groups of kno extra array or O(1) extra space allowedswap pairs of nodessingly linked list, only .next available

Template

function reverseList(head) {
    let prev = null;
    let curr = head;
    while (curr) {
        const next = curr.next;
        curr.next = prev;
        prev = curr;
        curr = next;
    }
    return prev;
}

Looks similar, isn't

Two pointers (same direction): Same-direction pointers walk indices over an array, one reading fast and one writing slow. Reversal has no array and no indices; it rewires the .next field on each node in place, which is pointer surgery, not index walking.

n up to 1e5 nodes -> O(n): one pass touching each node's pointer once, O(1) extra space.

Learn this pattern