Scale Engineer

Peeking Iterator

Med
#0271Time: O(n) for initialization, O(1) for peek(), next(), and hasNext() operationsSpace: O(n) where n is the number of elements in the iterator1 company
Patterns
DesignIterator
Data structures
Array
Companies
Google

Prompt

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

2 approaches with complexity analysis and trade-offs.

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.

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()

Walkthrough

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.

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

Complexity

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

Trade-offs

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

Solutions

// 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 (); } }

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