Question

Formatted question description: https://leetcode.ca/all/206.html

206. Reverse Linked List

Level

Easy

Description

Reverse a singly linked list.

Example:

Input: 1->2->3->4->5->NULL

Output: 5->4->3->2->1->NULL

Follow up:

A linked list can be reversed either iteratively or recursively. Could you implement both?

Solution

The recursive solution is to do the reverse operation for head.next, and set head.next.next = head and head.next = null.

The iterative solution uses two pointers, prev and curr. Initially, prev is null and curr points to head. Each step, let nextTemp be the next node of curr before reversing curr. Set curr.next = prev. After that, set both prev and curr to the next step (that is, prev = curr and curr = nextTemp). The loop ends when all nodes in the original list are reversed.

Code

Java

public class Reverse_Linked_List {

    /**
     * Definition for singly-linked list.
     * public class ListNode {
     *     int val;
     *     ListNode next;
     *     ListNode(int x) { val = x; }
     * }
     */

    /*
        1,2,3,4,5
        2,1 prev=2 - 3,4,5 current=3
        3,2,1 prev=3 - 4,5 current=4
        ...
     */
    // https://leetcode.com/problems/reverse-linked-list/solution/
    class Solution_oj_iterative {
        public ListNode reverseList(ListNode head) {
            ListNode prev = null;
            ListNode curr = head;
            while (curr != null) {
                ListNode nextTemp = curr.next;
                curr.next = prev;
                prev = curr;
                curr = nextTemp;
            }
            return prev;
        }
    }
    class Solution_oj_recursively {
        public ListNode reverseList(ListNode head) {
            if (head == null || head.next == null) return head;
            ListNode p = reverseList(head.next);
            head.next.next = head;
            head.next = null;
            return p;
        }
    }

    class Solution_recursively {
        public ListNode reverseList(ListNode head) {

            ListNode dummy = new ListNode(0);
            // dummy.next = head;

            ListNode current = head;

            reverse(dummy, current);

            return dummy.next;
        }

        private void reverse(ListNode dummy, ListNode current) {

            if (current == null) {
                return;
            }

            ListNode newHead = current.next;
            ListNode oldDummyNext = dummy.next;

            dummy.next = current;
            current.next = oldDummyNext;

            current = newHead;

            this.reverse(dummy, current);
        }
    }


    class Solution_iteratively {
        public ListNode reverseList(ListNode head) {

            ListNode dummy = new ListNode(0);
            // dummy.next = head;

            ListNode current = head;
            while (current != null) {

                ListNode newHead = current.next;
                ListNode oldDummyNext = dummy.next;

                dummy.next = current;
                current.next = oldDummyNext; // initial node, oldDummyNext is null, which is what we want, and which is why "// dummy.next = head;" is commented out above

                current = newHead;
            }

            return dummy.next;
        }
    }
}

// OJ: https://leetcode.com/problems/reverse-linked-list/
// Time: O(N)
// Space: O(1)
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        ListNode h;
        while (head) {
            auto p = head;
            head = head->next;
            p->next = h.next;
            h.next = p;
        }
        return h.next;
    }
};

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
  def reverseList(self, root):
    if not root or not root.next:
      return root

    ret = self.reverseList(root.next)
    root.next.next = root
    root.next = None
    return ret

  def _reverseList(self, head):
    pre = None
    cur = head
    while cur:
      tmp = cur.next
      cur.next = pre
      pre = cur
      cur = tmp
    return pre

  # iteratively as queue head inserting
  def __reverseList(self, head):
    """
    :type head: ListNode
    :rtype: ListNode
    """
    dHead = dummy = ListNode(-1)
    p = head
    while p:
      tmp = dummy.next
      dummy.next = p
      p = p.next
      dummy.next.next = tmp
    return dHead.next

  # easily leads to a circle. Remove current node's next after recursive call.
  def ___reverseList(self, head):
    self.newHead = None

    def rec(head):
      if not head:
        return head
      p = rec(head.next)
      head.next = None
      if p:
        p.next = head
      else:
        self.newHead = head
      return head

    rec(head)
    return self.newHead

206 - Reverse Linked List