148-Sort-List.c

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2) {
    if (NULL == l1)
    {
        return l2;
    }
    if (NULL == l2)
    {
        return l1;
    }
    struct ListNode *head = NULL, *tail = NULL;
    while (NULL != l1 && NULL != l2)
    {
        if (l1->val < l2->val)
        {
            if (NULL == head)
            {
                head = l1;
                tail = l1;
            }
            else
            {
                tail->next = l1;
                tail = tail->next;
            }
            l1 = l1->next;
        }
        else
        {
            if(NULL == head)
            {
                head = l2;
                tail = l2;
            }
            else
            {
                tail->next = l2;
                tail = tail->next;
            }
            l2 = l2->next;
        }
    }
    if (NULL != l1)
    {
        tail->next = l1;
    }
    if (NULL != l2)
    {
        tail->next = l2;
    }
    return head;
}

struct ListNode* sortList(struct ListNode* head) {
    if (NULL == head || NULL == head->next)
    {
        return head;
    }
    struct ListNode *temp, *lhs, *rhs;
    if (NULL == head->next->next)
    {
        if (head->val > head->next->val)
        {
            temp = head;
            head = head->next;
            head->next = temp;
            temp->next = NULL;
        }
        return head;
    }
    struct ListNode *slow = head, *fast = head;
    while (NULL != fast && NULL != fast->next)
    {
        slow = slow->next;
        fast = fast->next->next;
    }
    temp = slow->next;
    slow->next = NULL;
    lhs = sortList(head);
    rhs = sortList(temp);
    return mergeTwoLists(lhs, rhs);
}