2019-01-22 Day17

1. 反转一个单链表。

示例:

输入: 1->2->3->4->5->NULL
输出: 5->4->3->2->1->NULL

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        
        if( head == NULL ) return head;
        ListNode * listhead = new ListNode( 0 );
        listhead -> next = head;
        ListNode * first , *second , *third ;
        first = listhead; second = head ; third = head -> next;
        
        while(third)
        {
            second -> next = first;
            first = second;
            second = third;
            third = third -> next;
           
        }
        second -> next = first;
        head -> next = NULL ;
        return second ;
    }
};