[LeetCode]Sort List

/**
 * 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) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        int len = getLen(head);
        return sortListUtil(head, len);
    }
private:
    ListNode* sortListUtil(ListNode* head, int len)
    {
        if(len <= 1) return head;
        ListNode* head1 = NULL;
        ListNode* head2 = NULL;
        int len1, len2;
        divideList(head, len, head1, len1, head2, len2);
        head1 = sortListUtil(head1, len1);
        head2 = sortListUtil(head2, len2);
        ListNode* newHead = mergeList(head1, head2);
        return newHead;
    }
    int getLen(ListNode* head)
    {
        int len = 0;
        while(head)
        {
            len++;
            head = head->next;
        }
        return len;
    }
    void divideList(ListNode* head, int len, ListNode*& head1, int& len1, ListNode*& head2, int& len2)
    {
        int firstHalfLen = len/2;
        head1 = head;
        ListNode* pDivideNode = head1;
        for(int k = 1; k < firstHalfLen; ++k)
            pDivideNode = pDivideNode->next;
        if(pDivideNode != NULL)
        {
            head2 = pDivideNode->next;
            pDivideNode->next = NULL;
        }
        else head2 = NULL;
        len1 = firstHalfLen;
        len2 = len-len1;
    }
    ListNode* mergeList(ListNode* head1, ListNode* head2)
    {
        ListNode dummy(-1);
        ListNode* prev = &dummy;
        while(head1 && head2)//all not null
        {
            if(head1->val < head2->val)
            {
                prev->next = head1;
                head1 = head1->next;
            }
            else
            {
                prev->next = head2;
                head2 = head2->next;
            }
            prev = prev->next;
        }
        //remained head1 is not null
        if(head1) prev->next = head1;
        //remained head2 is not null
        if(head2) prev->next = head2;
        
        return dummy.next;
    }
};