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.

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