Reverse a singly linked list.
Hint:
A linked list can be reversed either iteratively or recursively. Could you implement both?
使用两个变量分别记录当前节点的前一个节点和后一个节点，当遍历链表的时候，将当前节点的 next 节点修改为前一个节点。
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
class Solution {
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;
}
}