题解 | #反转链表#

/**
 * struct ListNode {
 *  int val;
 *  struct ListNode *next;
 *  ListNode(int x) : val(x), next(nullptr) {}
 * };
 */
class Solution {
  public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param head ListNode类
     * @return ListNode类
     */
    ListNode* ReverseList(ListNode* head) {
        // write code here
        if(head==nullptr)
        {
            return nullptr;
        }
        ListNode* p = head;
        while (p != nullptr) {
            p = p->next;
        }
        ListNode* q, *r;
        while (head->next != p) {
            q = head;
            r = head->next;
            while (r != p) {
                int temp = r->val;
                r->val = q->val;
                q->val = temp;
                q = r;
                r = r->next;
            }
            p = q;
        }
        return head;
    }
};