Reverse a singly linked list.

一刷
题解：在每次的iter中，会有3个节点变量。prev, cur, next

/**
 * 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 dummy = new ListNode(-1);
        dummy.next = head;
        ListNode cur = head.next, prev = head;
        while(cur!=null){
            ListNode next = cur.next;
            cur.next = prev;
            if(prev == head) prev.next = null;
            prev = cur;
            dummy.next = cur;
            cur = next;
        }
        
        return dummy.next;
    }
}

更简洁一点?

/**
 * 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) {
       ListNode newHead = null;
        while(head!=null){
            ListNode next = head.next;
            head.next = newHead;
            newHead = head;
            head = next;
        }
        return newHead;
    }
}

三刷

/**
 * 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) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode prev = null, next;
        while(head!=null){
            next = head.next;
            head.next = prev;
            dummy.next = head;
            prev = head;
            head = next;
        }
        
        return dummy.next;
    }
}