反转链表
每次反转相邻的两个指针的指向即可
每一次让q位置的指针指向p
∅ 5 4 3 2 1 ∅
p q
p q
............
p q 结束循环
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* reverseList(ListNode* head) {
ListNode* p = nullptr,* q = head;
if (!q) return q;
while (q)
{
auto r = q -> next;
q -> next = p;
p = q;
q = r;
}
return p;
}
};