23 Merged K sorted lists

• 第一个方法，brute force，把所有elements收集到arraylist，然后sort，然后存到linked list。

public ListNode mergeKLists(ListNode[] lists) {
    List<Integer> l = new ArrayList<Integer>();
    //存到数组
    for (ListNode ln : lists) {
        while (ln != null) {
            l.add(ln.val);
            ln = ln.next;
        }
    }
    //数组排序
    Collections.sort(l);
    //存到链表
    ListNode head = new ListNode(0);
    ListNode h = head;
    for (int i : l) {
        ListNode t = new ListNode(i);
        h.next = t;
        h = h.next;
    }
    h.next = null;
    return head.next;
}

随后像merged two SOrted Lists一样，两辆合并。但是也有小技巧，s.h.i.t

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
    ListNode h = new ListNode(0);
    ListNode ans=h;
    while (l1 != null && l2 != null) {
        if (l1.val < l2.val) {
            h.next = l1;
            h = h.next;
            l1 = l1.next;
        } else {
            h.next = l2;
            h = h.next;
            l2 = l2.next;
        }
    }
    if(l1==null){
        h.next=l2;
    }
    if(l2==null){
        h.next=l1;
    } 
    return ans.next;
}
public ListNode mergeKLists(ListNode[] lists) {
    if(lists.length==1){
        return lists[0];
    }
    if(lists.length==0){
        return null;
    }
    ListNode head = mergeTwoLists(lists[0],lists[1]);//返回的总是排好序的第一个节点
    for (int i = 2; i < lists.length; i++) {
        head = mergeTwoLists(head,lists[i]);//然后和后一个开始交♂换
    }
    return head;
}