1 // Java Iterator interface reference:
 2 // https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
 3 class PeekingIterator implements Iterator<Integer> {
 4     Iterator<Integer> itr;
 5     boolean peeked;
 6     int peekVal;
 7 
 8     public PeekingIterator(Iterator<Integer> iterator) {
 9         // initialize any member here.
10         this.itr = iterator;
11     }
12 
13     // Returns the next element in the iteration without advancing the iterator.
14     public Integer peek() {
15         if(peeked) {
16             return peekVal;
17         } else {
18             peeked = true;
19             peekVal = itr.next();
20         }
21         return peekVal;
22     }
23 
24     // hasNext() and next() should behave the same as in the Iterator interface.
25     // Override them if needed.
26     @Override
27     public Integer next() {
28         if(peeked) {
29             peeked = false;
30             return peekVal;
31         } else return itr.next();
32     }
33 
34     @Override
35     public boolean hasNext() {
36         return peeked || itr.hasNext();
37     }
38 }