An interesting problem about iterators. shares a very nice solution, which is rewritten below, with minor simplifications.

// 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
     
      & 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
      
       & nums) : Iterator(nums) {        // Initialize any member here.        // **DO NOT** save a copy of nums and manipulate it directly.        // You should only use the Iterator interface methods.        peeked = false;    }    // Returns the next element in the iteration without advancing the iterator.    int peek() {        if (!peeked) {            peeked = true;            peekElem = Iterator::next();        }        return peekElem;    }    // hasNext() and next() should behave the same as in the Iterator interface.    // Override them if needed.    int next() {        if(peeked) {            peeked = false;            return peekElem;        }        return Iterator::next();    }    bool hasNext() const {        return peeked || Iterator::hasNext();    }private:    bool peeked;    int peekElem;};
      
     

BTW, it seems that we tend to misspell peek to peak. Well, after learning what peek wants to do, I understood why it uses such a name: peek means to look furtively, which is just like what peek does compared to next :-)

This post shares another super concise solution, just 3 lines of added code in total! And I just know too few about classes in C++ to come up with it...

// 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
     
      & 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
      
       & nums) : Iterator(nums) {        // Initialize any member here.        // **DO NOT** save a copy of nums and manipulate it directly.        // You should only use the Iterator interface methods.            }    // Returns the next element in the iteration without advancing the iterator.    int peek() {        return Iterator(*this).next();    }    // hasNext() and next() should behave the same as in the Iterator interface.    // Override them if needed.    int next() {        return Iterator::next();    }    bool hasNext() const {        return Iterator::hasNext();    }};