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PeekingIterator.java

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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> {

private Iterator<Integer> iterator;

private Integer nextElement = null;

private boolean hasPeeked = false;

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 == true) {

return nextElement;

}

nextElement = iterator.next();

hasPeeked = true;

return nextElement;

}

// hasNext() and next() should behave the same as in the Iterator interface.

// Override them if needed.

@Override

public Integer next() {

if (hasPeeked == false) {

return iterator.next();

}

hasPeeked = false;

Integer result = nextElement;

nextElement = null;

return result;

}

@Override

public boolean hasNext() {

return hasPeeked || iterator.hasNext();

}

}