LeetCode 206 反转列表

[海超人and大洋游侠]

 递归法

/**
 * 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* answer = nullptr;
        while(head != nullptr){
            answer = new ListNode(head->val,answer);
            head = head->next;
        }
        return answer;
    }
};

三个指针法

/**
 * 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* first = head;
        ListNode* second = nullptr;
        while(first != nullptr) {
            ListNode* next = first->next;
            first->next = second;
            second = first;
            first = next;
        }
        return second;
    }
};