source-code/Peeking_Iterator.cpp

// Below is the interface for Iterator, which is already defined for you.
// **DO NOT** modify the interface for Iterator.
class Iterator {
    struct Data;
	Data* data;
public:
	Iterator(const vector<int>& nums);
	Iterator(const Iterator& iter);
	virtual ~Iterator();
	// Returns the next element in the iteration.
	int next();
	// Returns true if the iteration has more elements.
	bool hasNext() const;
};


class PeekingIterator : public Iterator {
public:
	PeekingIterator(const vector<int>& nums) : Iterator(nums), hasPeeked(false) {
	    
	}

    // Returns the next element in the iteration without advancing the iterator.
	int peek() {
        if(!hasPeeked) {
            peekedElement = Iterator::next();
            hasPeeked = true;
        }
        return peekedElement;
	}

	// hasNext() and next() should behave the same as in the Iterator interface.
	// Override them if needed.
	int next() {
	    if(!hasPeeked) {
	        return Iterator::next();
	    }
	    int result = peekedElement;
	    hasPeeked = false;
	    return result;
	}

	bool hasNext() const {
	    return hasPeeked or Iterator::hasNext();
	}
private:
    bool hasPeeked;
    int peekedElement;
};