104-合并k个排序链表

2017.10.16.终于有时间继续刷刷题了。

果然一做链表的题逻辑很清楚，就是这个写起来就很乱。

k个链表就是两两的合并就好了。

/**
 * Definition for ListNode.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int val) {
 *         this.val = val;
 *         this.next = null;
 *     }
 * }
 */ 
public class Solution {
    /**
     * @param lists: a list of ListNode
     * @return: The head of one sorted list.
     */
  public ListNode mergeKLists(List<ListNode> lists) {  
        // write your code here
		if(lists.size() == 0){
			return null;
		}
		ListNode res = lists.get(0);
		if(lists.size() == 1){
			return res;
		}
		for(int i = 1; i < lists.size() ; i++){
			ListNode tmp = lists.get(i);
			res = merger(res,tmp);
		}
		return res;
    }
	public ListNode merger(ListNode l1,ListNode l2){
		if(l1 == null){
			return l2;
		}
		if(l2 == null){
			return l1;
		}
		ListNode flag = new ListNode(-1);//头结点的位置
		ListNode res = flag;//插入的位置
		while(l1 != null && l2 != null){
			if(l1.val < l2.val){
				flag.next = l1;
				flag = flag.next;
				l1 = l1.next;
			}
			else{
				flag.next = l2;
				flag = flag.next;
				l2 = l2.next;
			}
		}
		while(l1 != null){
			flag.next = l1;
			flag = flag.next;
			l1 = l1.next;
		}
		while(l2 != null){
			flag.next = l2;
			flag = flag.next;
			l2 = l2.next;
		}
		return res.next;
	}
}