## Algorithm

Problem Name: 284. Peeking Iterator

Design an iterator that supports the `peek` operation on an existing iterator in addition to the `hasNext` and the `next` operations.

Implement the `PeekingIterator` class:

• `PeekingIterator(Iterator<int> nums)` Initializes the object with the given integer iterator `iterator`.
• `int next()` Returns the next element in the array and moves the pointer to the next element.
• `boolean hasNext()` Returns `true` if there are still elements in the array.
• `int peek()` Returns the next element in the array without moving the pointer.

Note: Each language may have a different implementation of the constructor and `Iterator`, but they all support the `int next()` and `boolean hasNext()` functions.

Example 1:

```Input
["PeekingIterator", "next", "peek", "next", "next", "hasNext"]
[[[1, 2, 3]], [], [], [], [], []]
Output
[null, 1, 2, 2, 3, false]

Explanation
PeekingIterator peekingIterator = new PeekingIterator([1, 2, 3]); // [1,2,3]
peekingIterator.next();    // return 1, the pointer moves to the next element [1,2,3].
peekingIterator.peek();    // return 2, the pointer does not move [1,2,3].
peekingIterator.next();    // return 2, the pointer moves to the next element [1,2,3]
peekingIterator.next();    // return 3, the pointer moves to the next element [1,2,3]
peekingIterator.hasNext(); // return False
```

Constraints:

• `1 <= nums.length <= 1000`
• `1 <= nums[i] <= 1000`
• All the calls to `next` and `peek` are valid.
• At most `1000` calls will be made to `next`, `hasNext`, and `peek`.

## Code Examples

### #1 Code Example with C++ Programming

```Code - C++ Programming```

``````
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:
deque < int>q;
public:
PeekingIterator(const vector<int>& nums) : Iterator(nums) {}

// Returns the next element in the iteration without advancing the iterator.
int peek() {
if(q.empty()) q.push_front(Iterator::next());
return q.front();
}

// hasNext() and next() should behave the same as in the Iterator interface.
// Override them if needed.
int next() {
if(!q.empty()){
int t = q.front();
q.pop_front();
return t;
}
return Iterator::next();;
}

bool hasNext() const {
return Iterator::hasNext() || !q.empty();
}
};
``````
Copy The Code &

Input

cmd
["PeekingIterator", "next", "peek", "next", "next", "hasNext"] [[[1, 2, 3]], [], [], [], [], []]

Output

cmd
[null, 1, 2, 2, 3, false]

### #2 Code Example with Java Programming

```Code - Java Programming```

``````
class PeekingIterator implements Iterator {
private Iterator iterator;
private Integer topElement;

public PeekingIterator(Iterator < Integer> iterator) {
// initialize any member here.
this.iterator = iterator;
populateTopElement();
}

// Returns the next element in the iteration without advancing the iterator.
public Integer peek() {
return this.topElement;
}

// hasNext() and next() should behave the same as in the Iterator interface.
// Override them if needed.
@Override
public Integer next() {
Integer nextValue = this.topElement;
populateTopElement();
return nextValue;
}

@Override
public boolean hasNext() {
return this.topElement != null;
}

private void populateTopElement() {
this.topElement = this.iterator.hasNext() ? this.iterator.next() : null;
}
}
``````
Copy The Code &

Input

cmd
["PeekingIterator", "next", "peek", "next", "next", "hasNext"] [[[1, 2, 3]], [], [], [], [], []]

Output

cmd
[null, 1, 2, 2, 3, false]

### #3 Code Example with Python Programming

```Code - Python Programming```

``````
class PeekingIterator:
def __init__(self, iterator):
self.it = iterator
self.bool = self.it.hasNext()
self.num =  self.it.next() if self.bool else None
def peek(self):
return self.num

def next(self):
n = self.num
self.bool = self.it.hasNext()
if self.bool:
self.num = self.it.next()
return n

def hasNext(self):
return self.bool
``````
Copy The Code &

Input

cmd
["PeekingIterator", "next", "peek", "next", "next", "hasNext"] [[[1, 2, 3]], [], [], [], [], []]

Output

cmd
[null, 1, 2, 2, 3, false]