题目：

题解：

class PeekingIterator implements Iterator {     private Iterator iterator;     private E nextElement;      public PeekingIterator(Iterator iterator) {         this.iterator = iterator;         nextElement = iterator.next();     }          public E peek() {         return nextElement;     }          @Override     public E next() {         E ret = nextElement;         nextElement = iterator.hasNext() ? iterator.next() : null;         return ret;     }          @Override     public boolean hasNext() {         return nextElement != null;     } }