24. 反转链表

解题思路

递归

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

迭代

使用头插法。

public ListNode ReverseList(ListNode head) {
    ListNode newList = new ListNode(-1);
    while (head != null) {
        ListNode next = head.next;
        head.next = newList.next;
        newList.next = head;
        head = next;
    }
    return newList.next;
}