链表定义

class ListNode {
    int val;
    ListNode next;

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

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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;
    }

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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;
    }

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