sortList.cpp

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
private:
    ListNode* findMiddleNode(ListNode* head) {
        ListNode* slow = head;
        ListNode* fast = head->next;
        
        while(fast != NULL && fast->next != NULL) {
            fast = fast->next->next;
            slow = slow->next;
        }
        ListNode* mid = slow->next;
        slow->next = NULL;
        
        return mid;
    }
    
    ListNode* mergeList(ListNode* l1, ListNode* l2) {
        ListNode* dummyHead = new ListNode(-1);
        ListNode* current = dummyHead;
        
        while(l1 != NULL && l2 != NULL) {
            if(l1->val <= l2->val) {
                current->next = l1;
                l1 = l1->next;
            } else {
                current->next = l2;
                l2 = l2->next;
            }
            
            current = current->next;
        }
        
        current->next = l1 != NULL ? l1 : l2;
        
        return dummyHead->next;
    }
public:
    ListNode* sortList(ListNode* head) {
        if(head == NULL || head->next == NULL)
            return head;
        
        ListNode* middle = findMiddleNode(head);
        
        ListNode* left = sortList(head);
        ListNode* right = sortList(middle);
        
        return mergeList(left, right);
    }
};