# Two AC Solutions both in C#.

• public class Solution {
public ListNode MergeKLists(ListNode[] lists)
{
// Solution1
//var result = MergeKListsHelper(lists, 0, lists.Length - 1);
//return result;

``````        // Solution2
ListNode dummy = new ListNode(0);
ListNode temp = dummy;
int removeHeadIndex = -1;

while(lists.Any(l => l != null))
{
ListNode min = new ListNode(Int32.MaxValue);
for(int i =0;i<lists.Length;i++)
{
if(lists[i] != null && lists[i].val<min.val)
{
min = lists[i];
}
}

temp.next = min;
temp = temp.next;
}

return dummy.next;
}
private ListNode MergeKListsHelper(ListNode[] lists, int start, int end)
{
if (start == end) return lists[start];
if( start<end)
{
int mid = start + (end - start) / 2;// 不要用(start+end)/2 容易超过int的范围
ListNode l1 = MergeKListsHelper(lists, start, mid);
ListNode l2 = MergeKListsHelper(lists, mid + 1, end);
var result = MergeTwo(l1, l2);
return result;
}

return null;
}
private ListNode MergeTwo(ListNode l1, ListNode l2)
{
if (l1 == null)
return l2;
if (l2 == null)
return l1;
ListNode dummy = new ListNode(0);
ListNode cur = dummy;
while (l1 != null && l2 != null)
{
if (l1.val < l2.val)
{
cur.next = l1;
l1 = l1.next;
}
else
{
cur.next = l2;
l2 = l2.next;
}
cur = cur.next;
}

if (l1 != null)
{
cur.next = l1;
}
if (l2 != null)
{
cur.next = l2;
}
return dummy.next;
}
``````

}

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