LeetCode OJ-- Sort List **@

链表排序，要求使用 O(nlgn) 时间，常量空间。

使用归并的思路

/**
 * 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;
    }
};