我使用了一个 TreeSet的＆LT;整数GT; ，我想很简单，喜欢找一些在设置索引。有没有一个很好的办法做到这一点，实际上是利用二叉树的O（日志（N））的复杂性？

I'm using a TreeSet<Integer> and I'd quite simply like to find the index of a number in the set. Is there a nice way to do this that actually makes use of the O(log(n)) complexity of binary trees?

（如果没有，我该怎么办，而且没有人知道为什么不呢？我很好奇，为什么这样一类会不会像搜索功能包含在Java的。）

(If not, what should I do, and does anyone know why not? I'm curious why such a class would be included in Java without something like a search function.)

推荐答案

由于@Yrlec指出 set.headSet（元素）.size 将返回0，虽然没有该元素在集。所以，我们最好检查：

As @Yrlec points out set.headSet(element).size will returns 0 though there is no this element in the set. So we'd better check:

 return set.contains(element)? set.headSet(element).size(): -1;

下面是一个测试用例，显示问题：

Here is a test case to show the problem:

public static void main(String args[]){
    TreeSet<Integer> set = new TreeSet<>();
    set.add(4);
    set.add(2);
    set.add(3);
    set.add(1);

    System.out.println(set.headSet(1).size());//0
    System.out.println(set.headSet(2).size());//1
    System.out.println(set.headSet(3).size());//2
    System.out.println(set.headSet(4).size());//3
    System.out.println(set.headSet(-1).size());//0!!Caution!,retusn 0 though it does not exits

}