#Leetcode# 148. Sort List

https://leetcode.com/problems/sort-list/ 

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

Example 1:

Input: 4->2->1->3
Output: 1->2->3->4

Example 2:

Input: -1->5->3->4->0
Output: -1->0->3->4->5

代码：

/**
 * 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(!head || !head -> next) return head;
        ListNode *fast = head, *slow = head, *pre = head;
        while(fast && fast -> next) {
            pre = slow;
            slow = slow -> next;
            fast = fast -> next -> next;
        }
        pre -> next = NULL;
        return mergesort(sortList(head), sortList(slow));
    }
    ListNode *mergesort(ListNode* l1, ListNode* l2) {
        ListNode *dummy = new ListNode(-1);
        ListNode *cur = dummy;
        while(l1 && l2) {
            if(l1 -> val < l2 -> val) {
                cur -> next = l1;
                l1 = l1 -> next;
            } else {
                cur -> next = l2;
                l2 = l2 -> next;
            }
            
            cur = cur -> next;
        }
        
        if(l1) cur -> next = l1;
        if(l2) cur -> next = l2;
        return dummy -> next;
    }
};

　　归并排序 学到了

FHFHFH