148. Sort List

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* sortList(ListNode* head) {
        return sortList(head, NULL);
    }
    ListNode* sortList(ListNode* head, ListNode* tail) {
        if (head == tail || head->next == tail) return head;
        ListNode *halfhead = middleList(head, tail);
        ListNode *h1 = sortList(head, halfhead);
        ListNode *h2 = sortList(halfhead, tail);
        return mergeList(h1, halfhead, h2, tail);
    }
    ListNode* middleList(ListNode* head, ListNode* tail) {
        ListNode *fast = head, *slow = head;
        while (fast != tail && fast->next != tail) {
            fast = fast->next->next;
            slow = slow->next;
        }
        return slow;
    }
    ListNode* mergeList(ListNode* h1, ListNode* t1, ListNode* h2, ListNode* t2) {
        ListNode h(0);
        ListNode *p = &h;
        while (h1 != t1 && h2 != t2) {
            if (h1->val > h2->val) {
                p->next = h2;
                h2 = h2->next;
            }
            else {
                p->next = h1;
                h1 = h1->next;
            }
            p = p->next;
        }
        while (h1 != t1) {
            p->next = h1;
            h1 = h1->next;
            p = p->next;
        }
        while (h2 != t2) {
            p->next = h2;
            h2 = h2->next;
            p = p->next;
        }
        p->next = t2;  // don't forget to set last node's next to t2
        return h.next;
    }
};