反转链表

public ListNode reverseList(ListNode head) {
    ListNode prev = null;
    ListNode curr = head;
    while (curr != null) {
        ListNode next = curr.next;
        curr.next = prev;
        prev = curr;
        curr = next;
    }
    return prev;
}

public class ListNode {
    int val;
    ListNode next;

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

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

public class ListNode {
    int val;
    ListNode next;

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