合并k个升序链表(leetcode23)

public class leetcode23 {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        ListNode item1= new ListNode(1);
        item1.next = new ListNode(4);
        item1.next.next = new ListNode(5);
        
        ListNode item2 = new ListNode(1);
        item2.next = new ListNode(3);
        item2.next.next = new ListNode(4);
        
        ListNode item3 = new ListNode(2);
        item3.next = new ListNode(6);
        
        
        ListNode head = mergeKLists(new ListNode[]{item1,item2,item3});
        head.printNode(head);
    }
    //用一个变量 ans 来维护以及合并的链表，第 i 次循环把第 i 个链表和 ans 合并，答案保存到 ans 中。
    public static ListNode mergeKLists(ListNode[] lists){
        ListNode ans = null;
        for(int i=0;i<lists.length;++i){
            ans = mergeTwoLists(ans,lists[i]);
        }
        return ans;
    }
    //合并两个链表
    public static ListNode mergeTwoLists(ListNode a,ListNode b){
        if(a==null||b==null){
            return a!=null?a:b;
        }
        
        ListNode head = new ListNode(0);
        ListNode tail = head,aPtr = a,bPtr = b;
        while(aPtr!=null&&bPtr!=null){
            if(aPtr.val<bPtr.val){
                tail.next = aPtr;
                aPtr = aPtr.next;
            }else{
                tail.next = bPtr;
                bPtr = bPtr.next;
            }
            tail = tail.next;
        }
        
        tail.next = (aPtr!=null?aPtr:bPtr);
        return head.next;
    }

}