腾讯//排序链表

在 O(n log n) 时间复杂度和常数级空间复杂度下，对链表进行排序。

示例 1:

输入: 4->2->1->3
输出: 1->2->3->4

示例 2:

输入: -1->5->3->4->0
输出: -1->0->3->4->5

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* findMiddle(ListNode* head){
        if(head!=nullptr&&head->next==nullptr)
            return head;
        if(head!=nullptr&&head->next!=nullptr&&head->next->next==nullptr)
            return head;
        ListNode* fast = head;
        ListNode* slow = head;
        while(fast->next!=nullptr&&fast->next->next!=nullptr){
            fast = fast->next->next;
            slow = slow->next;
        }
        return slow;
    }
    ListNode* merge(ListNode* first, ListNode* second){
        ListNode* head = new ListNode(0);
        head->next = first->val<=second->val?first:second;
        ListNode* tail = head;
        while(first!=nullptr&&second!=nullptr){
            if(first->val<=second->val){
                tail->next=first;
                tail=first;
                first=first->next;
            }else{
                tail->next=second;
                tail=second;
                second=second->next;
            }
        }
        if(first!=nullptr)
            tail->next=first;
        if(second!=nullptr)
            tail->next=second;
        return head->next;
    }
    ListNode* mergeSort(ListNode* head){
        if(head->next==nullptr)
            return head;
        auto mid = findMiddle(head);
        auto mid_next = mid->next;
        mid->next = nullptr;
        auto first = mergeSort(head);
        auto second = mergeSort(mid_next);
        return merge(first, second);
    }
    
    ListNode* sortList(ListNode* head) {
        if(head==nullptr || head->next==nullptr)
            return head;
        return mergeSort(head);
    }
};