Leetcode 147. Insertion Sort List

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.

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

Examples:

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

思路: 依照插入排序的方法，两层循环，外层遍历没有排序的节点，内层寻找该节点的位置，即从头遍历已排序的部分，直到值大于当前节点的值，连接到其前面的节点pre上。

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution(object):
    def insertionSortList(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if not head: return head
        if not head.next: return head
        
        p = head.next
        rh = ListNode(0)
        rh.next = ListNode(head.val)
        while p:
            r = rh.next
            pre = rh
            while r and p.val >= r.val:
                pre = r
                r = r.next
            s = p.next
            pre.next = p
            p.next = r
            p = s
        return rh.next

日期: 2020-11-28 加油呀！