Priority Queue：
常用方法：
1.offer（E e） Inserts the specified element into this priority queue
- poll ()
Retrieves and removes the head of this queue, or returns null if this queue is empty.
About override a new comparator:
new Comparator<ListNode>() {
public int compare\(ListNode node1, ListNode node2\) {
return node1.val - node2.val;
}
a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.