LeetCode: Insertion Sort List

LeetCode: Insertion Sort List

Sort a linked list using insertion sort.

地址：https://oj.leetcode.com/problems/insertion-sort-list/

算法：按题目的意思，采用插入排序对链表进行排序，时间复杂度为$O(n^2)$。代码：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *insertionSortList(ListNode *head) {
        ListNode *p = head, *q = NULL, *pre = NULL;
        if(p){
            p = p->next;
            head->next = NULL;
        }
        while(p){
            pre = NULL;
            q = head;
            while(q && q->val <= p->val){
                pre = q;
                q = q->next;
            }
            if(pre){
                pre->next = p;
                p = p->next;
                pre->next->next = q;
            }else{
                head = p;
                p = p->next;
                head->next = q;
            }
        }
        return head;
    }
};