list - Sort List

一，Sort List

题目描述

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

 

我的代码

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* getMiddleNodeOfList(ListNode* head){
        ListNode* slow = head;
        ListNode* fast = head;
        if(head == NULL){
            return NULL;
        }
        while(fast->next != NULL && fast->next->next != NULL) {
            slow = slow->next;
            fast = fast->next->next;
        }
        return slow;
    }

    ListNode* sortList(ListNode* head) {
        if(head == NULL || head->next == NULL) {
            return head;
        }
        ListNode *middle = getMiddleNodeOfList(head);
        ListNode *next   = middle->next;
        middle->next = NULL;
        return mergeList(sortList(head), sortList(next));
    }

    ListNode* mergeList(ListNode *list1, ListNode *list2){
        ListNode *head = new ListNode(-1);
        ListNode *p1, *p2, *pre = head;
        p1 = list1;
        p2 = list2;
        head->next = NULL;
        while(p1 != NULL && p2 != NULL){
            if(p1->val < p2->val){
                pre->next = p1;
                pre = p1;
                p1 = p1->next;
            } else {
                pre->next = p2;
                pre = p2;
                p2 = p2->next;
            }
        }
        if(p1 != NULL){
            pre->next = p1;
        }
        if(p2 != NULL){
            pre->next = p2;
        }
        return head->next;
    }
};