Merge k Sorted Lists

歸併排序在鏈表中的應用

Code:

//Runtime: 66 ms
ListNode *mergeTwoLists(ListNode *l1, ListNode *l2){
		if (l1 == NULL)	return l2;
		if (l2 == NULL)	return l1;
		ListNode *p = l1->val > l2->val ? l2 : l1;
		ListNode *prev = NULL;
		while (l1 != NULL && l2 != NULL){
			if (l1->val <= l2->val){
				if (prev != NULL){ prev->next = l1; }
				prev = l1;
				l1 = l1->next;
			}
			else{
				if (prev != NULL){ prev->next = l2; }
				prev = l2;
				l2 = l2->next;
			}
		}
		if (l2 == NULL){ prev->next = l1; }
		else if (l1 == NULL){ prev->next = l2; }
		return p;
	}
	ListNode *mergeKLists(vector<ListNode *> &lists) {
		if (lists.size() == 0)	return NULL;
		if (lists.size() == 1)	return lists[0];
		else{
			vector<ListNode*> left, right;
			int mid = lists.size() >> 1;
			for (int i = 0; i < mid; ++i)	left.push_back(lists[i]);
			for (int i = mid; i < lists.size(); ++i)	right.push_back(lists[i]);
			ListNode *l1 = mergeKLists(left);
			ListNode *l2 = mergeKLists(right);
			return mergeTwoLists(l1, l2);
		}
	}