284. Peeking Iterator

本来的Iterator类里面是自带next()和hasNext()的，但是现在加了peek()之后会调用一次iterator.next(),然后iterator的顺序就乱了。

所以一旦被peek过我们就需要把这个被next()弹出的元素记录下来，如果再调用next就返回这个临时的元素，再调用peek就还返回这个元素，再调用hasNext就返回有没有被peek过或者hasNext()本身

class PeekingIterator implements Iterator<Integer> {
private Iterator<Integer> it;
private Integer next;
private boolean isPeeked;

public PeekingIterator(Iterator<Integer> iterator) {
// initialize any member here.
it = iterator;
isPeeked = false;
}

// Returns the next element in the iteration without advancing the iterator.
public Integer peek() {
if (isPeeked) {
return next;
} else {
isPeeked = true;
next = it.next();
return next;
}
}

// hasNext() and next() should behave the same as in the Iterator interface.
// Override them if needed.
@Override
public Integer next() {
Integer res;
if(isPeeked) {
res = next;
} else {
res = it.next();
}
isPeeked = false;
return res;
}

@Override
public boolean hasNext() {
return isPeeked? true: it.hasNext();
}
}

bug记录：

peek()本身的时候也要检查有没有被peek过，否则会一直往后面看去，就会丢失元素