147. Insertion Sort List**

147. Insertion Sort List**

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

Title Description

Sort a linked list using insertion sort.

Algorithm of Insertion Sort:

1. Insertion sort iterates, consuming one input element each repetition, and growing a sorted output list.
2. At each iteration, insertion sort removes one element from the input data, finds the location it belongs within the sorted list, and inserts it there.
3. It repeats until no input elements remain.

Example 1:

Input: 4->2->1->3
Output: 1->2->3->4

Example 2:

Input: -1->5->3->4->0
Output: -1->0->3->4->5

C ++ implementation 1

Use dummy virtual nodes, saved already sorted node order is descending, and finally reverse the result.

pDummy initialized, and then with each node of current interest ptr, and pcomparing the next node pointed to, if they are suitable location, can be ptrinserted into pand p->nextbetween.

class Solution {
private:
    ListNode* reverse(ListNode *head) {
        ListNode *prev = nullptr;
        while (head) {
            auto tmp = head->next;
            head->next = prev;
            prev = head;
            head = tmp;
        }
        return prev;
    }
public:
    ListNode* insertionSortList(ListNode* head) {
        if (!head || !head->next) return head;
        ListNode *dummy = new ListNode(0);
        dummy->next = head;
        auto ptr = head->next;
        head->next = nullptr; // head 加入到 dummy 链表后, 要注意和 next 节点断开
        auto p = dummy;
        while (ptr) { // ptr 指向当前要访问的元素, 它要和 dummy 链表已存在的节点进行对比
            auto tmp = ptr->next;
            while (p->next && p->next->val > ptr->val) // 将较小的元素排在 dummy 链表的后面, 最后进行 reverse
                p = p->next;
            ptr->next = p->next;
            p->next = ptr;
            ptr = tmp; // ptr 接着指向下一个节点
            p = dummy; // p 恢复原位, dummy 链表的起始位置
        }
        return reverse(dummy->next);
    }
};