148. Sort List -- 时间复杂度O(n log n)

struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};

ListNode *sortList(ListNode *head) {
    if (head==NULL || head->next == NULL){
        return head;
    }
    //find the middle place
    ListNode *p1=head, *p2=head->next; 

    while(p2 && p2->next){
        p1 = p1->next;
        p2 = p2->next->next;
    }
    p2 = p1->next;
    p1->next = NULL;
    return mergeTwoLists(sortList(head), sortList(p2));
}


ListNode *mergeTwoLists(ListNode* head1, ListNode* head2){
    ListNode *p1 = head1,  *p2=head2;
    static ListNode dummy(0);

    ListNode *tail = &dummy;

    while(p1 && p2){
        if(p1->val < p2->val){
            tail->next = p1;
            p1 = p1->next;
        }else{
            tail->next = p2;
            p2 = p2->next;
        }
        tail = tail->next;
    }
    if (p1) tail->next = p1;
    if (p2) tail->next = p2;

    return dummy.next;
}