src/main/java/com/leetcode/design/medium/PeekingIterator_284.java
public Integer peek() {
      if (next == null) {
        throw new NoSuchElementException();
      }
      return next;
    }
// Returns the next element in the iteration without advancing the iterator.
Returns the next element in the iteration without advancing the iterator.
Returns the next element in the iteration without advancing the iterator .
public Integer peek() {
      if (next == null) {
        throw new NoSuchElementException();
      }
      return next;
    }
public Integer peek ( ) { if ( next == null ) { throw new NoSuchElementException ( ) ; } return next ; }
Returns the next element in the iteration without advancing the iterator.
Returns the next element in the iteration without advancing the iterator .