题解 | 反转链表

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 *   public ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param head ListNode类
     * @return ListNode类
     */
    public ListNode ReverseList (ListNode head) {
        // write code here
        ListNode ptr = head;
        ListNode newList = new ListNode(-1);
        ListNode toInsert;
        while (ptr != null) {
            toInsert = ptr;
            ptr = ptr.next;
            toInsert.next = null;
            insert(newList, toInsert);
        }
        return newList.next;
    }
    public static void insert(ListNode root, ListNode toInsert) {
        toInsert.next = root.next;
        root.next = toInsert;
    }
}