# 1286. Iterator for Combination

<https://leetcode.com/problems/iterator-for-combination>

## Description

Design the `CombinationIterator` class:

* `CombinationIterator(string characters, int combinationLength)` Initializes the object with a string `characters` of **sorted distinct** lowercase English letters and a number `combinationLength` as arguments.
* `next()` Returns the next combination of length `combinationLength` in **lexicographical order**.
* `hasNext()` Returns `true` if and only if there exists a next combination.

**Example 1:**

```
**Input**
["CombinationIterator", "next", "hasNext", "next", "hasNext", "next", "hasNext"]
[["abc", 2], [], [], [], [], [], []]
**Output**
[null, "ab", true, "ac", true, "bc", false]
**Explanation**
CombinationIterator itr = new CombinationIterator("abc", 2);
itr.next();    // return "ab"
itr.hasNext(); // return True
itr.next();    // return "ac"
itr.hasNext(); // return True
itr.next();    // return "bc"
itr.hasNext(); // return False
```

**Constraints:**

* `1 <= combinationLength <= characters.length <= 15`
* All the characters of `characters` are **unique**.
* At most `104` calls will be made to `next` and `hasNext`.
* It's guaranteed that all calls of the function `next` are valid.

## ac

```java
```