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反转链表
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;
 *     struct ListNode *next;
 * };
 */


struct ListNode* reverseList(struct ListNode* head){
    struct ListNode *p, *q, ret;
    ret.next = NULL;
    p = head;
    while(p){
        q = p->next;
        p->next = ret.next;
        ret.next = p;
        p = q;
    }
    return ret.next;
}