Sort List

<-M 148> Sort List

/**
 * 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 {
public:
    ListNode* sortList(ListNode* head) {
        if(head == NULL || head->next == NULL)
            return head;
        ListNode *fast = head;
        ListNode *slow = head;
        while(fast->next != NULL && fast->next->next !=  NULL ) {
            fast = fast->next->next;
            slow = slow->next;
        }

        fast = slow;
        slow = slow->next;
        fast->next = NULL;

        ListNode *l1 = sortList(head);
        ListNode *l2 = sortList(slow);
        return mergeTwoList(l1, l2);
    }
private:
    ListNode *mergeTwoList(ListNode *l1, ListNode *l2) {
        ListNode check(-1);

        for(ListNode *p = ✓ l1 != nullptr || l2 != nullptr; p = p->next) {
            int val1 = l1 == nullptr ? INT_MAX : l1->val;
            int val2 = l2 == nullptr ? INT_MAX : l2->val;
            if(val1 <= val2) {
                p->next = l1;
                l1 = l1->next;
            }
            else {
                p->next = l2;
                l2 = l2->next;
            }
        }
        return check.next;
    }
};