题解 | #反转链表#

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/

//一、迭代
public class Solution {
  public ListNode ReverseList(ListNode head) {
    ListNode l = null;
    ListNode p = head;
    while (p != null) {
      ListNode r = p.next;
      p.next = l;
      l = p;
      p = r;
    }
    return l;
  }
}

//二、递归
public class Solution {
  public ListNode ReverseList(ListNode head) {
    if (head == null || head.next == null) {
      return head;
    }
    ListNode p = ReverseList(head.next);
    head.next.next = head;
    head.next = null;
    return p;
  }
}