Leetcode Insertion Sort List

Sort a linked list using insertion sort.

思路就跟数组的插入排序思想一致，但是因为链表的只能从前往后遍历且无法随机访问，所以操作比较复杂，需要小心处理。

代码如下：

/**
* 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* shead = new ListNode(0x80000000);
ListNode* cur = head,*next;
while(cur)
{
next = cur->next;
ListNode* sCur = shead->next;
ListNode* pre = shead;
while(sCur)
{
if(cur->val > sCur->val)
{
pre = pre->next;
sCur = sCur->next;
}
else
break;
}
cur->next = pre->next;
pre->next = cur;
cur = next;
}
return shead->next;
}
};

结果发现性能不够理想，看看前面的更快的代码。发现其实不需要每次都进行从头判断进行插入，如果新插入的元素的值大于已经有序的元素中的最大值，直接进行下一个元素的判断即可，代码如下：

/**
 * 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* extra = new ListNode(0);
        extra->next = head;
        ListNode* p = extra;
        while(p->next){
            auto cur = p->next;
            if(p == extra || cur->val >= p->val)
            {
                p = p->next;
                continue;
            }
            else
            {
                p->next = cur->next;
            }
            
            auto pre = extra;
            while(pre->next->val < cur->val)
                pre = pre->next;
            cur->next = pre->next;
            pre->next = cur;
        }
        return extra->next;
    }
};