Sort List.cpp

/**
 * 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) {
        return mergeSort(head);
    }
    
private:
    ListNode* mergeSort(ListNode* head)
    {
        if(!head || !head->next)return head;
        //partition
        auto fast = head, slow = head;
        while(fast->next && fast->next->next)
        {
            fast = fast->next->next;
            slow = slow->next;
        }
        auto head2 = slow->next;
        slow->next = nullptr;
        auto curr1 = mergeSort(head), curr2 = mergeSort(head2);
        
        //merge
        ListNode* newHead = curr1->val <= curr2->val? curr1: curr2, *curr = newHead;
        if(curr == curr1)curr1 = curr1->next;
        else curr2 = curr2->next;
        while(curr1 && curr2)
        {
            if(curr1->val <= curr2->val)
            {
                curr->next = curr1;
                curr = curr->next;
                curr1 = curr1->next;
            }
            else
            {
                curr->next = curr2;
                curr = curr->next;
                curr2 = curr2->next;
            }
        }
        if(curr1)curr->next = curr1;
        if(curr2)curr->next = curr2;
        return newHead;
    }
};