206. 反转链表

一、Java

class Solution {
    public ListNode reverseList(ListNode head) {
        if (head == null) return null;
        ListNode pre = null, cur = head, next = head.next;
        while (next != null) {
            cur.next = pre;
            pre = cur;
            cur = next;
            next = cur.next;
        }
        cur.next = pre;
        return cur;
    }
}

class Solution {
    public ListNode reverseList(ListNode head) {
        ListNode pre = null, cur = head, next;
        while (cur != null) {
            next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }
        return pre;
    }
}

class Solution {
    public ListNode reverseList(ListNode head) {
        if (head == null || head.next == null) return head;
        ListNode next = head.next, newHead = reverseList(next);
        head.next = null;
        next.next = head;
        return newHead;
    }
}

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