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In this approach, we precompute all the combinations of the given string of a specified length. This allows us to provide the next combination in constant time by just iterating over the precomputed list. For this, we can use recursion or libraries to generate combinations initially and store them in an internal data structure.
To find all combinations of a particular length, you can use backtracking or an iterative process that resembles a binary-like increase in combinations selection.
Time Complexity: O(n!) for precomputation, O(1) for next and hasNext.
Space Complexity: O(n!) due to storage of combinations.
1from itertools import combinations
2
3class CombinationIterator:
4 def __init__(self, characters: str, combinationLength: int):
5 self.combinations = list(combinations(characters, combinationLength))
6 self.index = 0
7
8 def next(self) -> str:
9 comb = self.combinations[self.index]
10 self.index += 1
11 return ''.join(comb)
12
13 def hasNext(self) -> bool:
14 return self.index < len(self.combinations)This Python solution uses the itertools.combinations library to generate all the combinations of the given length at the initialization stage. Whenever next() is called, it returns the current combination and advances the index.
Instead of generating all combinations at once, we can generate them on the fly when next() is called. This approach uses bit manipulation to generate the combination efficiently by representing each combination as a bitmask. However, this requires understanding how to generate valid combinations incrementally with proper bit manipulation.
Time Complexity: O(1) for next and hasNext, average complexity due to combination generation logic.
Space Complexity: O(combinationLength), the storage used for the indices.
1class CombinationIterator {
2 constructor(
This JavaScript solution generates combinations during the initialization phase using a depth-first search approach. The _generateCombinations method is used to recursively create all combinations. The next method fetches the combinations sequentially from the precomputed list.