反转链表

leetcode 206

    //迭代
    public ListNode reverseList(ListNode head) {
        if(head == null || head.next == null) return head;
        ListNode pre = head,cur = head.next,nex;
        pre.next = null;
        while(cur!=null){
            nex = cur.next;
            cur.next = pre;
            pre = cur;
            cur = nex;
        }
        return pre;
    }
        //递归
    public ListNode reverseList(ListNode head) {
        ListNode pre = null,cur = head;
        return recur(pre,cur);
    }
    private ListNode recur(ListNode pre,ListNode cur){
        if(cur == null) return pre;
        ListNode nex = cur.next;
        cur.next = pre;
        pre = cur;
        return recur(pre,nex);
    }
    //递归2
    public ListNode reverseList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode p = reverseList(head.next);
        head.next.next = head;
        head.next = null;
        return p;
    }