LeetCode OJ-- Sort List **@

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鏈表排序，要求使用 O(nlgn) 時間，常量空間。

使用歸併的思路

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */class Solution {public:    ListNode *findMid(ListNode *head)    {        ListNode *mid;        ListNode *fast = head->next;        ListNode *slow = head;        while(fast && fast->next)        {            fast = fast->next;            fast = fast->next;            slow = slow->next;        }        mid = slow;        return mid;    }    ListNode *sortList(ListNode *head) {        if(head == NULL || head->next == NULL)            return head;        ListNode *tmp = findMid(head);        ListNode *right = sortList(tmp->next);        tmp->next = NULL;        ListNode *left = sortList(head);                ListNode *newHead = new ListNode(-1);        newHead->next = merge(left,right);        return newHead->next;    }    ListNode *merge(ListNode *left, ListNode *right)    {        ListNode *dummy = new ListNode(-1);        for(ListNode *p = dummy; left!=NULL || right!= NULL; p = p->next)        {            int leftVal = left == NULL? INT_MAX:left->val;            int rightVal = right == NULL?INT_MAX:right->val;            if(leftVal <= rightVal)            {                p->next = left;                left = left->next;            }            else            {                p->next = right;                right = right->next;            }        }        return dummy->next;    }};