算法基础题：归并排序

题目描述

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

采用归并排序

/**
 * 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) {
        //合法性检验
        if(!head || !head->next)
            return head;
        //快慢指针找中点
        ListNode *p1 = head, *p2 = head->next;
        while(p2 && p2->next)
        {
            p1 = p1->next;//慢指针
            p2 = p2->next->next; //快指针
        }
        //p1此时就是中点
        
        ListNode* right = sortList(p1->next);//递归右边中点
        p1->next = NULL;
        ListNode* left = sortList(head);//递归左边中点
        
        return merge(left, right);
    }
    
    ListNode *merge(ListNode *left, ListNode *right) {
        //创建临时节点
        ListNode temp(0);
        ListNode *p = &temp;
        while(left && right) {
            //较小的置于临时节点后
            if(left->val < right->val)
            {
                p->next = left;
                left = left->next;
            }
            else {
                p->next = right;
                right = right->next;
            }
            p = p->next;
        }
        if (left) 
            p->next = left;
        
        if (right)
            p->next = right;
        
        return temp.next;
    }
};