LeetCode之“链表”：Sort List

　　题目链接

　　题目要求：

　　Sort a linked list in O(n log n) time using constant space complexity.

　　满足O(n log n)时间复杂度的有快排、归并排序、堆排序。在这里采用的是归并排序（空间复杂度O(log n)），具体程序如下：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* merge(ListNode* l1, ListNode* l2)
    {
        ListNode *dummy = new ListNode(0);
        ListNode *head = nullptr, *start = dummy;
        while(l1 && l2)
        {
            if(l1->val < l2->val)
            {
                start->next = l1;
                l1 = l1->next;
            }
            else 
            {
                start->next = l2;
                l2 = l2->next;
            }
            start = start->next;
        }
        
        if(!l1)
            start->next = l2;
        else if(!l2)
            start->next = l1;
        
        head = dummy->next;
        delete dummy;
        dummy = nullptr;
        
        return head;
    }
    
    ListNode* mergeSort(ListNode* head)
    {
        if(!head->next)
            return head;
        
        ListNode *slow = head, *fast = head;
        ListNode *preNode = slow;
        while(fast && fast->next)
        {
            preNode = slow;
            slow = slow->next;
            fast = fast->next->next;
        }
        preNode->next = nullptr;
        
        ListNode *left = mergeSort(head);
        ListNode *right = mergeSort(slow);
        return merge(left, right);
    }

    ListNode* sortList(ListNode* head) {
        if(!head || !head->next)
            return head;
            
        return mergeSort(head);
    }
};