206链表翻转

### 解题思路

利用双指针将链表进行翻转

### 代码

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(head == NULL) return NULL;
        ListNode* a = head;
        ListNode* b = a->next;
        while(b){
            auto c = b->next;
            b->next = a;
            a = b,b = c;
        }
        head->next = NULL;
        return a;
    }
};