LeetCode 206题 反转链表（Reverse Linked List）

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

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
       //1.基本问题的解
        if(head == null || head.next == null){
           return head;
       }
       //2.将大问题分解成小问题
        ListNode reve = reverseList(head.next);
        //3.将小问题的解变成大问题的解
        head.next.next = head;
        head.next = null;
        return reve;
    }
}