leetcode 1286. Iterator for Combination

Design an Iterator class, which has:

• A constructor that takes a string characters of sorted distinctlowercase English letters and a number combinationLength as arguments.
• A function next() that returns the next combination of length combinationLength in lexicographical order.
• A function hasNext() that returns True if and only if there exists a next combination.

python3:利用itertools.combinations模块

class CombinationIterator:

    def __init__(self, A: str, k: int):
        self.it = itertools.combinations(A, k)
        self.last = A[-k:]
        self.res = ""
        

    def next(self) -> str:
        self.res = ''.join(next(self.it))
        return self.res

    def hasNext(self) -> bool:
        return self.res != self.last

C++：先产生所有符合字典序的子字符串。(回溯法）

class CombinationIterator {
private:
    queue<string> iterator;
    void combinationIterator(const string &characters, const int &combinationLength, int startpos, string &oneInstance, queue<string> &resultQueue) {
        if (oneInstance.length() == combinationLength) {
            resultQueue.push(oneInstance);
            return;
        }
        for (int i = startpos; i < characters.length(); ++i) {
            oneInstance.push_back(characters[i]);
            combinationIterator(characters, combinationLength, i + 1, oneInstance, resultQueue);
            oneInstance.pop_back();
        }
    }
public:
    CombinationIterator(string characters, int combinationLength) {
        string oneInstance = "";
        combinationIterator(characters, combinationLength, 0, oneInstance, iterator);
    }
    
    string next() {
        string ans = "";
        if (hasNext()) {
            ans = iterator.front();
            iterator.pop();
        }
        return ans;
    }
    
    bool hasNext() {
        //队列非空，说明还有
        return !iterator.empty();
    }
};