## # 284. Peeking Iterator

Given an Iterator class interface with methods: next() and hasNext(), design and implement a PeekingIterator that support the peek() operation -- it essentially peek() at the element that will be returned by the next call to next().

Example:

Assume that the iterator is initialized to the beginning of the list: [1,2,3].

Call next() gets you 1, the first element in the list.
Now you call peek() and it returns 2, the next element. Calling next() after that still return 2.
You call next() the final time and it returns 3, the last element.
Calling hasNext() after that should return false.

Follow up: How would you extend your design to be generic and work with all types, not just integer?

### # Solution

Approach 1: Use one cache to hold the number for peeking.

Approach 2: If we are allowed to save a copy of nums.

### # Code (Python)

Approach 1:

# Note: the actual python iterator implementation does not have has_next() -- easier to ask for forgivenness than permission.
# Here's an implementation of has_next() -- the approach is very similar with what we have for this problem.
class MyIterator:
def __init__(self, iterator):
self.iterator = iterator
self._find_next()

def next(self):
if not self.has_next_item:
raise StopIteration
temp = self.next_item
self._find_next()
return temp

def has_next(self):
return self.has_next_item

def _find_next(self):
self.has_next_item = True
try:
self.next_item = next(self.iterator)
except StopIteration:
self.has_next_item = False

i = MyIterator(iter([1,2]))
print(i.has_next()) # True
print(i.has_next()) # True
print(i.next()) # 1
print(i.has_next()) # True
print(i.next()) # 2
print(i.has_next()) # False
print(i.has_next()) # False
print(i.next()) # StopIteration

Approach 2:

### # Code (C++)

Approach 1:

// 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 {
private:
int cache;
bool hasCache;
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.
hasCache = false;
}

// Returns the next element in the iteration without advancing the iterator.
int peek() {
if (!hasCache)
{
cache = next();
hasCache = true;
}
return cache;
}

// hasNext() and next() should behave the same as in the Iterator interface.
// Override them if needed.
int next() {
if (hasCache)
{
hasCache = false;
return cache;
}
else
{
return Iterator::next();
}
}

bool hasNext() const {
return hasCache || Iterator::hasNext();
}
};

Approach 2:

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() {
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();
}
};