单链表相关面试题--2.反转一个单链表

// 三个指针翻转的思想完成逆置
struct ListNode* reverseList(struct ListNode* head) {
    if(head == NULL || head->next == NULL)
        return head;
    
    struct ListNode* n1, *n2, *n3;
    n1 = head;
    n2 = n1->next;
    n3 = n2->next;
    n1->next = NULL;
    //中间节点不为空，继续修改指向
    while(n2)
    {
        //中间节点指向反转
        n2->next = n1;
        //更新三个连续的节点
        n1 = n2;
        n2 = n3;
        if(n3)
            n3 = n3->next;
    }
    //返回新的头
    return n1;
}

// 取节点头插的思想完成逆置
struct ListNode* reverseList(struct ListNode* head) {
    struct ListNode* newhead = NULL;
    struct ListNode* cur = head;
    while(cur)
    {
        struct ListNode* next = cur->next;
        //头插新节点，更新头
        cur->next = newhead;
        newhead = cur;
        cur = next;
    }
    
    return newhead;
}