leetcode 148. 排序链表

我的解题

Merge sort

#include <bits/stdc++.h>
using namespace std;
/**
 * 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) {}
 * };
 */

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* sortList(ListNode *head)
    {
        return sortList(head, nullptr);
    }
    /**
     * @brief 左闭右开
     *
     * @param head
     * @param tail
     * @return 执行依据排好序的linked list，它会创造新的linked list
     */
    ListNode* sortList(ListNode *head, ListNode *tail)
    {
        if (head == nullptr) // 为什么空指针也可以直接返回
        {
            return head;
        }
        if (head->next == tail) // 只剩下一个节点，因为是左闭右开
        {
            head->next = nullptr; // 断开链表
            return head;
        }
        ListNode *slow = head, *fast = head;
        while (fast != tail)
        {
            slow = slow->next;
            fast = fast->next;
            if (fast != tail)
            {
                fast = fast->next;
            }
        }
        ListNode *mid = slow;
        return merge(sortList(head, mid), sortList(mid, tail));
    }
    /**
     * @brief 合并两个linked list
     *
     * @param head1
     * @param head2
     * @return
     */
    ListNode* merge(ListNode *head1, ListNode *head2)
    {
        ListNode *dummyHead = new ListNode(0);
        ListNode *temp = dummyHead, *temp1 = head1, *temp2 = head2;
        while (temp1 != nullptr && temp2 != nullptr)
        {
            if (temp1->val <= temp2->val)
            {
                temp->next = temp1;
                temp1 = temp1->next;
            }
            else
            {
                temp->next = temp2;
                temp2 = temp2->next;
            }
            temp = temp->next;
        }
        if (temp1 != nullptr)
        {
            temp->next = temp1;
        }
        else if (temp2 != nullptr)
        {
            temp->next = temp2;
        }
        return dummyHead->next;
    }
};

int main()
{

}
// g++ test.cpp --std=c++11 -pedantic -Wall -Wextra -g