链表问题---单链表的选择排序

【题目】

　　给定一个无序链表的头节点head，实现单链表的选择排序。
　　要求：空间复杂度O(1)。

【基本思路】

　　选择排序的过程是从未排序的部分中找到最小值，然后放在已经排好序部分的尾部，逐渐将未排序的部分缩小，最后全部变成排好序的部分。时间复杂度O( N2 )。

【代码实现】

#python3.5
def selectionSort(head):
def getSmallestPre(head):
if head == None:
return None
        pre = head    #important
        smallest = head
        smallPre = None
        head = head.next
while head != None:
if head.val < smallest.val:
                smallest = head
                smallPre = pre
            pre = head
            head = head.next
return smallPre


if head == None or head.next == None:
return head
    tail = None     #排序后链表的尾节点
    newHead = None
    cur = head
    small = None
while cur != None:
        smallPre = getSmallestPre(cur)    #最小节点的前一个节点，方便删除节点
if smallPre != None:
            small = smallPre.next
            smallPre.next = small.next
else:
            small = cur
            cur = cur.next
if tail == None:
            tail = small
            newHead = tail
else:
            tail.next = small
            tail = small
return newHead