No one sees Rover AI,except you

Thoughts

• Two pointers — prev (starts at null) and curr (starts at head)
• Save curr.next, then flip curr.next to point at prev
• Slide forward: prev = curr, curr = next. When curr is null, prev is the new head

Solution

Kotlin implementation:

fun reverseList(head: Node?): Node? {
    // walk once, flipping each next pointer
    var prev: Node? = null
    var curr = head

    while (curr != null) {
        // stash next before we overwrite the link
        val next = curr.next
        curr.next = prev

        // advance both pointers one step
        prev = curr
        curr = next
    }

    // prev now sits on the new head
    return prev
}