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