# Peeking Iterator
**Difficulty:** MEDIUM
[External](https://leetcode.com/problems/peeking-iterator)
Canonical: https://scaleengineer.com/dsa/problems/peeking-iterator
**Patterns:** [Design](https://scaleengineer.com/dsa/patterns/design), [Iterator](https://scaleengineer.com/dsa/patterns/iterator)
**Data structures:** Array
**Companies:** [Google](https://scaleengineer.com/companies/google)
---
## Problem
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`.

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

# Approaches
## Using Queue Data Structure
This approach uses a Queue data structure to store all elements from the iterator. While this works, it's not memory efficient as it stores all elements upfront.
**Time:** O(n) for initialization, O(1) for peek(), next(), and hasNext() operations · **Space:** O(n) where n is the number of elements in the iterator
**Pros:** Simple implementation; Easy to understand; O(1) time complexity for all operations after initialization
**Cons:** Uses extra space to store all elements; Not memory efficient; Copies all elements during initialization
### Explanation
In this approach, we initialize a Queue and store all elements from the iterator into it. This allows us to easily peek at the next element using the queue's peek operation. However, this approach uses extra space proportional to the size of the iterator.

```java
class PeekingIterator implements Iterator<Integer> {
    private Queue<Integer> queue;
    
    public PeekingIterator(Iterator<Integer> iterator) {
        queue = new LinkedList<>();
        while (iterator.hasNext()) {
            queue.offer(iterator.next());
        }
    }
    
    public Integer peek() {
        return queue.peek();
    }
    
    @Override
    public Integer next() {
        return queue.poll();
    }
    
    @Override
    public boolean hasNext() {
        return !queue.isEmpty();
    }
}
```

The implementation is straightforward:
1. In the constructor, we create a new queue and copy all elements from the iterator into it
2. peek() simply returns the first element in the queue without removing it
3. next() removes and returns the first element from the queue
4. hasNext() checks if the queue is not empty
### Algorithm
1. Initialize a Queue in constructor
2. Copy all elements from iterator to queue
3. For peek(): return queue.peek()
4. For next(): return queue.poll()
5. For hasNext(): return !queue.isEmpty()

## Caching Next Element
This approach maintains a single cached element to support the peek operation. It's more memory efficient as it only stores one element ahead.
**Time:** O(1) for all operations including initialization · **Space:** O(1) as it only stores one element
**Pros:** Memory efficient - only stores one element; O(1) time complexity for all operations; No upfront processing required; Can be easily extended to work with generic types
**Cons:** Slightly more complex implementation; Needs to handle null cases carefully
### Explanation
This approach keeps track of the next element by caching it. When peek() is called, we return the cached element. When next() is called, we return the cached element and cache the next element from the iterator.

```java
class PeekingIterator implements Iterator<Integer> {
    private Iterator<Integer> iterator;
    private Integer nextElement;
    
    public PeekingIterator(Iterator<Integer> iterator) {
        this.iterator = iterator;
        // Cache the first element if exists
        if (iterator.hasNext()) {
            nextElement = iterator.next();
        }
    }
    
    public Integer peek() {
        return nextElement;
    }
    
    @Override
    public Integer next() {
        Integer result = nextElement;
        nextElement = iterator.hasNext() ? iterator.next() : null;
        return result;
    }
    
    @Override
    public boolean hasNext() {
        return nextElement != null;
    }
}
```

The implementation works as follows:
1. In the constructor, we store the iterator and cache the first element if it exists
2. peek() returns the cached element without modifying any state
3. next() returns the cached element and updates the cache with the next element from the iterator
4. hasNext() checks if we have a cached element
### Algorithm
1. Store iterator and cache first element in constructor
2. For peek(): return cached element
3. For next(): return cached element and update cache
4. For hasNext(): check if cached element exists

# Solutions
### Java

```java
// Java Iterator interface reference: // https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html class PeekingIterator implements Iterator < Integer > { private Iterator < Integer > iterator ; private boolean hasPeeked ; private Integer peekedElement ; public PeekingIterator ( Iterator < Integer > iterator ) { // initialize any member here. this . iterator = iterator ; } // Returns the next element in the iteration without advancing the iterator. public Integer 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. @Override public Integer next () { if (! hasPeeked ) { return iterator . next (); } Integer result = peekedElement ; hasPeeked = false ; peekedElement = null ; return result ; } @Override public boolean hasNext () { return hasPeeked || iterator . hasNext (); } }
```

### CPP

```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); * * // 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. 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 (); hasPeeked = false ; return peekedElement ; } bool hasNext () const { return hasPeeked || Iterator :: hasNext (); } private: bool hasPeeked ; int peekedElement ; };
```

### Python

```python
# Below is the interface for Iterator, which is already defined for you. # # class Iterator: # def __init__(self, nums): # """ # Initializes an iterator object to the beginning of a list. # :type nums: List[int] # """ # # def hasNext(self): # """ # Returns true if the iteration has more elements. # :rtype: bool # """ # # def next(self): # """ # Returns the next element in the iteration. # :rtype: int # """ class PeekingIterator : def __init__ ( self , iterator ): """ Initialize your data structure here. :type iterator: Iterator """ self . iterator = iterator self . has_peeked = False self . peeked_element = None def peek ( self ): """ Returns the next element in the iteration without advancing the iterator. :rtype: int """ if not self . has_peeked : self . peeked_element = self . iterator . next () self . has_peeked = True return self . peeked_element def next ( self ): """ :rtype: int """ if not self . has_peeked : return self . iterator . next () result = self . peeked_element self . has_peeked = False self . peeked_element = None return result def hasNext ( self ): """ :rtype: bool """ return self . has_peeked or self . iterator . hasNext () # Your PeekingIterator object will be instantiated and called as such: # iter = PeekingIterator(Iterator(nums)) # while iter.hasNext(): # val = iter.peek() # Get the next element but not advance the iterator. # iter.next() # Should return the same value as [val].
```