Sort List--leetcode

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

由于问题要求复杂度为O(nlogn),所以按常规方法排序无法达到该复杂度，可采用归并排序实现。

设立两个快慢指针，找到中间节点位置作归并排序即可，实现代码如下：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *sortList(ListNode *head) {
        if (NULL == head || NULL == head->next)
            return head;
        ListNode *slow = head, *fast = head;
        while (slow && fast && fast->next && fast->next->next)
        {
            slow = slow->next;
            fast = fast->next->next;
        }
        fast = slow->next;
        slow->next = NULL;
        slow = head;
        slow = sortList(slow);
        fast = sortList(fast);
        
        return mergeTwoLists(slow, fast);
    }
    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2){
        ListNode *head = new ListNode(0);
        assert(head);
        ListNode *pCurr = head;
        
        while (l1 != NULL && l2 != NULL)
        {
            if (l1->val < l2->val)
            {
                pCurr->next = l1;
                l1 = l1->next;
            }
            else
            {
                pCurr->next = l2;
                l2 = l2->next;
            }
            pCurr = pCurr->next;
        }
        if (l1 != NULL)
            pCurr->next = l1;
        if (l2 != NULL)
            pCurr->next = l2;
        pCurr = head->next;
        delete head;
        head = pCurr;
        return head;
    }
};