Buttercola: Leetcode: Reverse Linked List

Sunday, August 30, 2015

Leetcode: Reverse Linked List

Reverse a singly linked list.

Hint:

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

Iterative solution:

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode reverseList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
         
        ListNode prev = null;
        ListNode curr = head;
        ListNode next = head.next;
         
        while (curr != null) {
            curr.next = prev;
            prev = curr;
            curr = next;
             
            if (next != null) {
                next = next.next;
            }
        }
         
        return prev;
    }
}

Recursive solution:

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode reverseList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
         
        ListNode next = head.next;
        ListNode newHead = reverseList(next);
         
        next.next = head;
        head.next = null;
         
        return newHead;
    }
}

Update on 11/6/15:
Recursive solution using two pointers:

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode reverseList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
         
        return reverseListHelper(null, head);
    }
     
    private ListNode reverseListHelper(ListNode prev, ListNode curr) {
        if (curr == null) {
            return prev;
        }
         
        ListNode newHead = reverseListHelper(curr, curr.next);
         
        curr.next = prev;
         
        return newHead;
    }
}

1 comment:

venkateshthallamOctober 22, 2017 at 11:00 PM
Though this is straightforward, I get confused with the extra pointer names, I found a easier way of naming things with code from here. Reverse a Single Linkedlist Java
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