284. Peeking Iterator - cocoder39/coco39_LC GitHub Wiki
Method 1: O(1) time and space complexity
the trick here is calling next()
before it is called by outside. Call next()
ahead inside constructor and next()
// 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) {
// Initialize any member here.
// **DO NOT** save a copy of nums and manipulate it directly.
// You should only use the Iterator interface methods.
//achieve peek() by calling next() ahead
my_hasNext = Iterator::hasNext();
if (my_hasNext)
my_next = Iterator::next();
}
// Returns the next element in the iteration without advancing the iterator.
int peek() { //return next val without advancing iterrator
return my_next;
}
// hasNext() and next() should behave the same as in the Iterator interface.
// Override them if needed.
int next() { //return next val with advancing iterator
//if stepping into this method
//then it is guaranteed that current my_next is true
//return previous val and call next() ahead
int t = my_next;
my_hasNext = Iterator::hasNext();
if (my_hasNext)
my_next = Iterator::next();
return t;
}
bool hasNext() const {
return my_hasNext;
}
private:
int my_next;
bool my_hasNext;
};
the trick is inside peek()
method where copy constructor is used
- make a copy first through copy constructor
- calling next on the copied iterator would not affect original iterator
// 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) {
// 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() {
//make a copy first through copy constructor
//calling next on the copied iterator would not affect original iterator
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();
}
};