Leetcode Reverse Linked List

Posted 2020-07-18 茜茜的技术空间

Reverse a singly linked list.

Hint:

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

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Java Code:

Iteratively: 

/**
 * 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 p1 = head;
        ListNode p2 = head.next;
        head.next = null;
        while(p1 != null && p2 != null) {
            ListNode p3 = p2.next;
            p2.next = p1;
            p1 = p2;
            if(p3!= null) {
                p2 = p3;
            }else {
                break;
            }
        }
        return p2;
    }
}

Recursively:

版本一：

 public ListNode reverseList(ListNode head) {
       if(head==null || head.next == null)
        return head;
 
        //get second node    
        ListNode second = head.next;
        //set first‘s next to be null
        head.next = null;
     
        ListNode rest = reverseList(second);
        second.next = head;
     
        return rest; 
    }

版本二：

 public ListNode reverseList(ListNode head) {
        return reverseListInt(head, null);
    }

    public ListNode reverseListInt(ListNode head, ListNode newHead) {
        if(head == null)
            return newHead;
        ListNode next = head.next;
        head.next = newHead;
        return reverseListInt(next, head);
    }

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20160601:

iteration:

public class Solution {
    public ListNode reverseList(ListNode head) {
     //iteration
        //base case
        if(head == null || head.next == null) {
            return head;
        }
        
        ListNode pre = null;
        ListNode cur = head;
        ListNode next = null;
        while(cur != null) {
            next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }
        return pre;
        }
}

recursion:

public class Solution {
    public ListNode reverseList(ListNode head) {
      //recursion
        //base case
        if(head == null || head.next == null) {
            return head;
        }
        ListNode nextNode = head.next;
        ListNode newHead = reverseList(nextNode);
        nextNode.next = head;
        head.next = null;
        return newHead;
    }
}

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Reference:

1. http://www.programcreek.com/2014/05/leetcode-reverse-linked-list-java/

2. https://leetcode.com/discuss/34474/in-place-iterative-and-recursive-java-solution