24. 反转链表

递归

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

迭代

头插法

public static ListNode reverseList(ListNode head) {
        ListNode newHead = null;
        ListNode current = head;
        while (current != null) {
            ListNode nextNode = current.next;
            current.next = newHead;
            newHead = current;
            current = nextNode;
        }
        return newHead;
    }