【原创】leetCodeOj --- Sort List 解题报告

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *sortList(ListNode *head) {
        if (!head)
            return NULL;
        int len = countLength(head);
        if (len == 1)
            return head;
        int mid = len / 2;
        ListNode *sec = dividedLst(head,mid);
        head = sortList(head);
        sec  = sortList(sec);
        head = mergeList(head,sec);
        return head;
    }
    
    ListNode *mergeList(ListNode *lst1,ListNode *lst2)
    {
        if (!lst2)
            return lst1;
        if (!lst1)
            return lst2;
        ListNode *ptr_lst1 = lst1;
        ListNode *ptr_lst2 = lst2;
        ListNode *begin = (ListNode *)malloc(sizeof(ListNode));
        ListNode *tail  = begin;
        tail->next = NULL;
        while (ptr_lst1 && ptr_lst2)
        {
            if (ptr_lst1->val < ptr_lst2->val)
            {
                tail->next = ptr_lst1;
                ptr_lst1   = ptr_lst1->next;
                tail->next->next = NULL;
                tail = tail->next;
            }
            else
            {
                tail->next = ptr_lst2;
                ptr_lst2   = ptr_lst2->next;
                tail->next->next = NULL;
                tail = tail->next;
            }
        }
        if (!ptr_lst1 && ptr_lst2) // lst1 is empty and lst2 has some
            tail->next = ptr_lst2;
        if (!ptr_lst2 && ptr_lst1) // lst1 has some and lst2 is empty
            tail->next = ptr_lst1;
        return begin->next;
    }
    
    int countLength(ListNode *head)
    {
        int count = 0;
        while (head)
        {
            count ++;
            head = head->next;
        }
        return count;
    }
    
    ListNode *dividedLst(ListNode *lst1,int mid)
    {
        ListNode *pre = lst1;
        int count = 0;
        while (lst1)
        {
            if (mid == count)
            {
                pre->next = NULL;
                return lst1;
            }
            count ++;
            pre = lst1;
            lst1 = lst1->next;
        }
    }
};