Sort List

来源：互联网发布：网络成瘾症如何治疗编辑：程序博客网时间：2024/06/05 11:26
Sort a linked list in O(n log n) time using constant space complexity.
/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */class Solution {public:    ListNode* findmid(ListNode* start, ListNode* end)    {        if(start == end)        {            return start;        }        if(start == NULL)        {            return NULL;        }        ListNode* slow = start;        ListNode* fast = start;        ListNode* mid = start;        while(1)        {            if(fast == end)            {                mid = slow;                break;            }            fast = fast -> next;            if(fast == end)            {                mid = slow;                break;            }            slow = slow -> next;            fast = fast -> next;        }        return mid;    }    ListNode* mergeSort(ListNode* start, ListNode* end)    {        if(start == end)        {            return start;        }        if(start == NULL)        {            return NULL;        }        ListNode* mid = findmid(start, end);         ListNode* tmp = mid -> next;        if(mid != NULL)        {            mid -> next = NULL;        }        ListNode* start1 = mergeSort(start, mid);        ListNode* start2 = mergeSort(tmp, end);        ListNode* head = NULL;        ListNode* tail = NULL;        if(start1 == NULL || start2 == NULL)        {            return start1 != NULL ? start1 : start2;        }        while(1)        {            if(start1 -> val < start2 -> val)            {                if(head == NULL)                {                    head = start1;                    tail = head;                    start1 = start1 -> next;                    tail -> next = NULL;                }else                {                    tail -> next = start1;                                    start1 = start1 -> next;                    tail = tail -> next;                                    tail -> next = NULL;                }            }else            {                if(head == NULL)                {                    head = start2;                    tail = head;                    start2 = start2 -> next;                    tail -> next = NULL;                }else                {                    tail -> next = start2;                                        start2 = start2 -> next;                    tail = tail -> next;                                        tail -> next = NULL;                }            }            if(start1 == NULL || start2 == NULL)            {                if(start1 == NULL)                {                    tail -> next = start2;                }                if(start2 == NULL)                {                    tail -> next = start1;                }                break;            }        }        return head;    }    ListNode* sortList(ListNode* head)    {        if(head == NULL)        {            return head;        }        ListNode* tail = head;        while(tail -> next != NULL)        {            tail = tail -> next;        }        return mergeSort(head, tail);    }};
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