206

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        ListNode *start, *end, *tmp;
        start = end = head;

        while(end && end->next) end = end->next;
        while(start != end){
            tmp = start;
            start = start->next;
            tmp->next = end->next;
            end->next = tmp;
        }
        return start;
    }
};