Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

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题目：归并 k 个有序链表，返回一个有序链表。

思路：之前有做过归并2个有序链表的题目，所以就想，将k个链表的归并简化成两两链表的归并。

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	public ListNode mergeKLists(List
  
    lists){
		if(lists == null)
			return null;
		return mergeLists(lists,0,lists.size()-1);
	}
	public ListNode mergeLists(List
   
     lists,int start,int end) { if(start > end) return null; if(start==end) return lists.get(start); if(start+1 == end) return mergeTwoLists(lists.get(start),lists.get(end)); int mid = (start+end)/2; return mergeTwoLists(mergeLists(lists,start,mid),mergeLists(lists,mid+1,end)); } public ListNode mergeTwoLists(ListNode l1, ListNode l2) { ListNode p1 = l1, p2 = l2, dummy = new ListNode(-1), p = dummy; while (p1 != null && p2 != null) { if (p1.val <= p2.val) { p.next = p1; p1 = p1.next; } else { p.next = p2; p2 = p2.next; } p = p.next; } if (p1 != null) p.next = p1; if (p2 != null) p.next = p2; return dummy.next; } // Definition for singly-linked list. public class ListNode { int val; ListNode next; ListNode(int x) { val = x; next = null; } }
   
  

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