206. Reverse Linked List

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LeetCode

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Problem

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?

Code

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

        ListNode pre = null;

        while(head != null){
            // 相当于改变了指向的方向
            // 1->2 变成了 1<-2
            ListNode temp = head.next;
            head.next = pre;
            pre = head;
            head = temp;
        }

        return pre;
    }
}