给你单链表的头节点 head ，请你反转链表，并返回反转后的链表。

一、迭代方法：

# java：
# Definition for singly-linked list.
 public class ListNode {
    int val;
    ListNode next;
    ListNode() {}
    ListNode(int val) { this.val = val; }
    ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 }
 
 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;
}

#python :
def reverseList(self, head: ListNode) -> ListNode:
    prev, curr = None, head
    while curr is not None:
        next = curr.next
        curr.next = prev
        prev = curr
        curr = next
    return prev

二、递归：

#java：
public ListNode reverseList(ListNode head) {
    // 1. 递归终止条件
    if (head == null || head.next == null) {
        return head;
    }
    ListNode p = reverseList(head.next);
    head.next.next = head;
    head.next = null;
    return p;
}

# python：
def reverseList(self, head: ListNode) -> ListNode:
    if head is None or head.next is None:
        return head
    
    p = self.reverseList(head.next)
    head.next.next = head
    head.next = None

    return p