206. 反转链表

1.递归反转
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        //递归法：我们直接来看递归最后一次（只有两个节点）的情况：tail = head
        //head.next.next = head;
        //head.next = null;
        //这样，每一次递归都返回反转链表的头节点。因此最后返回的就是整个链表的头节点
        //需要特判只有一个节点
        if (head == null || head.next == null)return head;
        ListNode tail = reverseList(head.next);
        head.next.next = head;
        head.next = null;
        return tail;
    }
}


2.双指针遍历
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        //使用两个指针来判断
        if (head == null || head.next == null)return head;
        ListNode curNode = head;
        ListNode nextNode = head.next;
        while(nextNode != null){
            ListNode temp = nextNode.next;
            nextNode.next = curNode;
            curNode = nextNode;
            nextNode = temp;
        }
        //最后尾指针要设定
        head.next = null;
        return curNode;
    }
}