Reverse Linked List

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* reverseList(struct ListNode* head) {
    if (head == NULL)
        return NULL;
    struct ListNode *tmp1 = head->next;    //这里使用了head->next；所以head == NULL判断必须放上面
    struct ListNode *tmp2 = head;
    
    if(head->next == NULL)
        return head;
    head = head->next;
    tmp2->next = NULL;
    while(head->next != NULL)
    {
        head = head->next; 
        tmp1->next = tmp2;
        tmp2 = tmp1;
        tmp1 = head;
    }
    head->next = tmp2;
    return head;
}