148.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 {
public:
    ListNode* merge(ListNode* left, ListNode* right){
        if(!left)
            return right;
        if(!right)
            return left;
        ListNode* res = new ListNode(0);
        ListNode* curr = res;
        while(left && right){
            if(left->val < right->val){
                curr->next = left;
                left = left->next;
            }
            else{
                curr->next = right;
                right = right->next;
            }
            curr = curr->next;
        }
        curr->next = (left!=NULL)?left:right;
        return res->next;
    }
    ListNode* sortList(ListNode* head) {
        if(!head || !head->next)
            return head;
        ListNode* slow = head;
        ListNode* fast = head;
        ListNode* prev = head;
        while(fast && fast->next){
            prev = slow;
            slow = slow->next;
            fast = fast->next->next;
        }
        prev->next = NULL;
        ListNode* left = sortList(head);
        ListNode* right = sortList(slow);
        return merge(left,right);
    }
};