111. Insertion Sort List

-147. Insertion Sort List
Total Accepted: 73736 Total Submissions: 246747 Difficulty: Medium

Sort a linked list using insertion sort.

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主要思路：
时间复杂度O(N)
空间复杂度O(1)
主要思路：逻辑上想象有两条链表
物理上还是一条
每次移动到一个元素将其插入已经有序的新链表

/** * 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) {        if(head==NULL) return NULL;        ListNode *p=head->next,*pnext,*prep=head;        head->next = NULL;        ListNode *q=head,*qnext,*tmp;        while(p){            q = head;            if(p->val<= q->val){//如果可以在最前面插入                    pnext  = p->next;                    p->next  = head;                    head = p;          //更新头部                    p = pnext;            }            else{                    while(q!=NULL){  //找到当前节点小于目标，下一节点大于目标                        qnext = q->next;                        if(qnext==NULL)break;                        if(qnext!=NULL&&qnext->val>p->val)break;                        q = qnext;                    }                    if(qnext==NULL) {//如果需要插入末尾                        q->next = p;                        p=p->next;                        q->next->next = NULL; //注意第二条链末尾置为空                    }                    else {    //如果是插入中间                        pnext = p->next;                        p->next = q->next;                        q->next  = p;                        p = pnext;                    }            }        }        return head;    }};