Description

Sort a linked list using insertion sort.

A graphical example of insertion sort. The partial sorted list (black) initially contains only the first element in the list.
With each iteration one element (red) is removed from the input data and inserted in-place into the sorted list

Algorithm of Insertion Sort:

Insertion sort iterates, consuming one input element each repetition, and growing a sorted output list.
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.
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

分析

题目的意思是：用插入排序来对链表排序。

• 这里用到了一个trick，用了额外变量dummy来实现，这样我们在处理头结点的时候就方便多了。

cpp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* dummy=new ListNode(-1);
        ListNode* cur;
        while(head){
            ListNode* t=head->next;
            cur=dummy;
            while(cur->next&&cur->next->val<head->val){
                cur=cur->next;
            }
            head->next=cur->next;
            cur->next=head;
            head=t;
        }
        return dummy->next;
    }
};

Python实现

下面是python的实现，也是借助了dummy的空节点：

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def insertionSortList(self, head: Optional[ListNode]) -> Optional[ListNode]:
        dummy = ListNode()
        curr = head

        while curr:
            next_p =  curr.next
            prev = dummy
            while prev.next and prev.next.val < curr.val:
                prev = prev.next
            curr.next = prev.next
            prev.next = curr
            
            curr = next_p
        return dummy.next

参考文献

[编程题]insertion-sort-list