Question

link

Implement the reversal of a singly linked list iteratively and recursively.

Iteratively

First, the iterative solution is very common, and is listed as one of the “5 fundamental operations of linked list” in the NineChap4 post. I will quote below.

First variant: Reverse from a particular node to the end

public ListNode reverse(ListNode start) {
    ListNode result = null;
    ListNode cur = start;
    while (cur != null) {
        ListNode temp = cur.next;
        cur.next = result;
        result = cur;
        cur = temp;
    }
    return result;
}

Second variant: Reverse from a node until another node

public ListNode reverseRange(ListNode start, int len) {
    ListNode result = null;
    ListNode cur = start;
    while (len > 0) {
        ListNode temp = cur.next;
        cur.next = result;
        result = cur;
        cur = temp;
        len--;
    }
    start.next = cur;
    return result;
}

Recursively

A good solution suggest by this answer:

public ListNode reverseRecursively(ListNode head) {
    if (head == null || head.next == null) {
        return head;
    }
    ListNode temp = head.next;
    // temp is not NULL
    ListNode newHead = reverseRecursively(temp);
    temp.next = head;
    head.next = null;
    return newHead;
}