LeetCode(2)——链表——反转链表

206. 反转链表

题目大意：
如题，反转链表

思路：
不开辟新空间，原地反转链表。

题解：

/**
 * 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) {
        if(head == nullptr || head->next == nullptr)
            return head;

        ListNode* curr = nullptr;
        ListNode* temp = new ListNode();
        while(head != nullptr)
        {
            temp = head;
            head = head->next;
            //temp->next = new ListNode();
            temp->next = curr;
            curr = temp;
        }

        return curr;
    }
};


 /*
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(head == nullptr || head->next == nullptr)
            return head;

        ListNode* curr_next = head;
        ListNode* temp = new ListNode();
        head = head->next;
        curr_next->next = nullptr;
        while(head != nullptr)
        {
            temp = head;
            head = head->next;
            //temp->next = new ListNode();
            temp->next = curr_next;
            curr_next = temp;
        }

        return curr_next;
    }
};
*/