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* getMid(ListNode* head)
    {
        ListNode* slow = head;
        ListNode* fast = head -> next;

        while(fast != NULL && fast -> next != NULL)
        {
            fast = fast -> next -> next;
            slow = slow -> next;
        }

        return slow;
    }

    ListNode* merge(ListNode* left, ListNode* right)
    {
        if(left == NULL)
            return right;
        if(right == NULL)
            return left;
        ListNode* ans = new ListNode(-1);
        ListNode* temp = ans;

        while(left != NULL && right != NULL)
        {
            if(right -> val < left -> val)
            {
                temp -> next = right;
                temp = right;
                right = right ->next;
            }
            else
            {
                temp -> next = left;
                temp = left;
                left = left -> next;
            }
        }

        while(left != NULL)
        {
            temp -> next = left;
            temp = left;
            left = left -> next;
        }
        while(right != NULL)
        {
            temp -> next = right;
            temp = right;
            right = right ->next;
        }

        return ans -> next;
    }

    ListNode* sortList(ListNode* head) {
        // base case
        if(head == NULL || head -> next == NULL)
            return head;

       //divide list into two halves
       ListNode* mid = getMid(head);

       ListNode* left = head;
       ListNode* right = mid -> next;
       mid -> next = NULL;

        //recursive calls for sort the both halves
       left = sortList(left);
       right = sortList(right);

       //merge the halves
       ListNode* ans = merge(left, right);

       return ans;
    }
};