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284. Peeking Iterator

  • Time: $O(1)$
  • Space: $O(n)$
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class PeekingIterator : public Iterator {
 public:
  PeekingIterator(const vector<int>& nums) : Iterator(nums) {}

  // Returns the next element in the iteration without advancing the iterator.
  int peek() {
    // Iterator(*this) makes a copy of current iterator, then call next on the
    // Copied iterator to get the next value without affecting current 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();
  }
};

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// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html

class PeekingIterator implements Iterator<Integer> {
  public PeekingIterator(Iterator<Integer> iterator) {
    this.iterator = iterator;
    buffer = iterator.hasNext() ? iterator.next() : null;
  }

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

  // hasNext() and next() should behave the same as in the Iterator interface.
  // Override them if needed.
  @Override
  public Integer next() {
    Integer next = buffer;
    buffer = iterator.hasNext() ? iterator.next() : null;
    return next;
  }

  @Override
  public boolean hasNext() {
    return buffer != null;
  }

  private Iterator<Integer> iterator;
  private Integer buffer;
}

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class PeekingIterator:
  def __init__(self, iterator: Iterator):
    self.iterator = iterator
    self.buffer = self.iterator.next() if self.iterator.hasNext() else None

  def peek(self) -> int:
    """
    Returns the next element in the iteration without advancing the iterator.
    """
    return self.buffer

  def next(self) -> int:
    next = self.buffer
    self.buffer = self.iterator.next() if self.iterator.hasNext() else None
    return next

  def hasNext(self) -> bool:
    return self.buffer is not None