284_peeking_iterator_java.md

Solution

// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
class PeekingIterator implements Iterator<Integer> {
    
    //This problem is given an iterator, we just need to adding a new feature "peek()"
    //We add a new filed "next" to accomplish that.
    
    private Integer next;
    private Iterator<Integer> itr;
    
    public PeekingIterator(Iterator<Integer> iterator) {
        // initialize any member here.
        this.itr = iterator;
        this.next = itr.hasNext() ? itr.next() : null;
        
    }

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

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

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

Analisys

This problem is to improve the Java's Iterator by implementing peek() method. We create two files Integer next and Iterator<Integer> itr to accomplish it. Remember to make fully use of this.next while implementing all its method. Notice that calling itr.next() inside the class next() function won't cause overflow exception, they are different methods in different classes (this.itr != this).