147. Insertion Sort List**

147. Insertion Sort List**

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

题目描述

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++ 实现 1

使用 dummy 虚拟节点, 保存已经排好序的节点, 顺序是从大到小, 最后 reverse 得到结果.

p 初始化为 dummy, 然后每次用当前关注的节点 ptr 和 p 所指向的下一个节点进行比较, 如果遇到合适的位置, 就可以将 ptr 插入到 p 和 p->next 之间.

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);
    }
};