leetcode - Sort List

 

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode * mergeList(ListNode * left, ListNode * right){
        ListNode * head = NULL, *lastNode, *curNode;
        while ((left != NULL) && (right != NULL)){
            if (left->val <= right->val){
                curNode = left;
                left = left->next;
            }
            else {
                curNode = right;
                right = right->next;
            }
            if (head == NULL)
                head = curNode;
            else lastNode->next = curNode;
            lastNode = curNode;
        }
        if (left != NULL){
            if (head == NULL)
                return left;
            else lastNode->next = left;
        }
        if (right != NULL){
            if (head == NULL)
                return right;
            else lastNode->next = right;
        }
        return head;
    }
    ListNode *sortList(ListNode *head) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        if (head == NULL || head->next == NULL)
            return head;
        ListNode * lhead = head, *rhead = head->next, *ltail = head;
        ListNode * itr = head->next;
        while ((itr != NULL) && (itr->next != NULL)){
            ltail = ltail->next;
            rhead = rhead->next;
            itr = (itr->next)->next;
        }
        ltail->next = NULL;
        return mergeList(sortList(lhead), sortList(rhead));
    }
};