341. Flatten Nested List Iterator

amortized O(1)
stack类似BST iterator

/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * class NestedInteger {
 *   public:
 *     // Return true if this NestedInteger holds a single integer, rather than a nested list.
 *     bool isInteger() const;
 *
 *     // Return the single integer that this NestedInteger holds, if it holds a single integer
 *     // The result is undefined if this NestedInteger holds a nested list
 *     int getInteger() const;
 *
 *     // Return the nested list that this NestedInteger holds, if it holds a nested list
 *     // The result is undefined if this NestedInteger holds a single integer
 *     const vector<NestedInteger> &getList() const;
 * };
 */
class NestedIterator {
public:
    NestedIterator(vector<NestedInteger> &nestedList) {
        s.assign(rbegin(nestedList), rend(nestedList));
    }

    int next() {
        int res = s.back().getInteger();
        s.pop_back(); // 别忘了pop
        return res;
    }

    bool hasNext() {
        while (!s.empty() && !s.back().isInteger()) {
            auto lst = s.back().getList();
            s.pop_back();
            s.insert(end(s), rbegin(lst), rend(lst));
        }
        return !s.empty();
    }

    vector<NestedInteger> s;
};

/**
 * Your NestedIterator object will be instantiated and called as such:
 * NestedIterator i(nestedList);
 * while (i.hasNext()) cout << i.next();
 */

/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * class NestedInteger {
 *   public:
 *     // Return true if this NestedInteger holds a single integer, rather than a nested list.
 *     bool isInteger() const;
 *
 *     // Return the single integer that this NestedInteger holds, if it holds a single integer
 *     // The result is undefined if this NestedInteger holds a nested list
 *     int getInteger() const;
 *
 *     // Return the nested list that this NestedInteger holds, if it holds a nested list
 *     // The result is undefined if this NestedInteger holds a single integer
 *     const vector<NestedInteger> &getList() const;
 * };
 */
class NestedIterator {
public:
    NestedIterator(vector<NestedInteger> &nestedList) {
        for (auto rit = nestedList.rbegin(); rit != nestedList.rend(); ++rit) {
            s.push(*rit);
        }
    }

    int next() {
        auto res = s.top().getInteger();
        s.pop();
        return res;
    }

    bool hasNext() {
        while (!s.empty() && !s.top().isInteger()) {
            auto lst = s.top().getList();
            s.pop();
            for (auto rit = lst.rbegin(); rit != lst.rend(); ++rit) {
                s.push(*rit);
            }
        }
        return !s.empty();
    }

    stack<NestedInteger> s;
};

/**
 * Your NestedIterator object will be instantiated and called as such:
 * NestedIterator i(nestedList);
 * while (i.hasNext()) cout << i.next();
 */

iterator版

/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * class NestedInteger {
 *   public:
 *     // Return true if this NestedInteger holds a single integer, rather than a nested list.
 *     bool isInteger() const;
 *
 *     // Return the single integer that this NestedInteger holds, if it holds a single integer
 *     // The result is undefined if this NestedInteger holds a nested list
 *     int getInteger() const;
 *
 *     // Return the nested list that this NestedInteger holds, if it holds a nested list
 *     // The result is undefined if this NestedInteger holds a single integer
 *     const vector<NestedInteger> &getList() const;
 * };
 */
class NestedIterator {
public:
    NestedIterator(vector<NestedInteger> &nestedList) {
        s.emplace(begin(nestedList), end(nestedList));
    }

    int next() {
        int res = s.top().first->getInteger();
        s.top().first++;
        return res;
    }

    bool hasNext() {
        while (!s.empty()) {
            auto &[b, e] = s.top();
            if (b == e) {
                s.pop();
            } else if (!b->isInteger()) {
                s.emplace(begin(b->getList()), end(b->getList()));
                ++b;
            } else break;
        }
        return !s.empty();
    }

    using vnii = vector<NestedInteger>::iterator;
    stack<pair<vnii, vnii>> s;
};

/**
 * Your NestedIterator object will be instantiated and called as such:
 * NestedIterator i(nestedList);
 * while (i.hasNext()) cout << i.next();
 */