[LeetCode] Insertion Sort List

Sort a linked list using insertion sort.

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 Linked List Sort

 

 

分析：把链表分成两部分：排好序的和为排序的，排好序的以NULL结尾，cur插在preInsertion和insertion之间，next保存cur->next.

        另外，注意dummy的使用简化了插入头结点的情况。

时间复杂度O(n^2),空间O(1) 

/**
 * 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)
        {   
            if(head == NULL)
                return NULL;

            ListNode dummy(INT_MIN);
            dummy.next = head;

            ListNode* preInsertion = NULL;
            ListNode* insertion = NULL;
            ListNode* cur = head->next;
            head->next = NULL;//break the sorted and unsorted
            ListNode* next = NULL;

            while(cur)
            {   
                preInsertion = & dummy;
                insertion = dummy.next;

                //find the right position
                while(insertion != NULL && cur->val >= insertion->val)
                {
                    preInsertion = preInsertion->next;
                    insertion = insertion->next;
                }

#if 0
                if(insertion == NULL)//cur is max, don't need to move it
                {
                    next = cur->next;//just store cur first;
                    preInsertion->next = cur; 
                    cur->next = NULL;
                    cur = next;
                }
                else
                {
                    next = cur->next;//just store cur first;
                    preInsertion->next = cur; 
                    cur->next = insertion;
                    cur = next;
                }
#endif
                next = cur->next;//just store cur first;
                preInsertion->next = cur;
                cur->next = insertion;
                cur = next;


            }
            return dummy.next;
        }
};