/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */

struct ListNode *reverseList(struct ListNode *head)
{
    struct ListNode *res = NULL;
    while (head)
    {
        struct ListNode *pre_node = head;
        head = head->next;
        pre_node->next = res;
        res = pre_node;
    }
    return res;
}