链表反转

链表定义

class ListNode {    int val;    ListNode next;    ListNode(int x) {        val = x;    }}

 

非递归实现很简单，只需要遍历一遍链表，在遍历过程中，把遍历的节点一次插入到头部。

public ListNode reverseList(ListNode head) {        ListNode prev = null;        while(head!=null){            ListNode tmp = head.next;            head.next = prev;            prev = head;            head = tmp;        }        return prev;    }

 

递归实现：翻转head->为首的链表， 然后head变为尾部节点

public ListNode reverseList(ListNode head) {        if(head==null||head.next ==null)            return head;        ListNode prev = reverseList(head.next);        head.next.next = head;        head.next = null;        return prev;    }