LeetCode Peeking Iterator

Description:

Given an Iterator class interface with methods:

next()

and

hasNext()

,
design and implement a PeekingIterator that support the

peek()

operation -- it essentially peek() at the
element that will be returned by the next call to next().

Here is an example. Assume that the iterator is initialized to the beginning of the list:

[1,
2, 3]

.

Call

next()

gets you 1, the first element in the list.

Now you call

peek()

and it returns 2, the next element. Calling

next()

after
that still return 2.

You call

next()

the final time and it returns 3, the last element. Calling

hasNext()

after
that should return false.

Solution:

开始没反应过来，后来发现只要建立一个类似于缓存机制的东西即可。

在这里，就用peekFlag和peekNum来代表是否已经peek过，已经peek的数值。

<span style="font-size:18px;">import java.util.*;

// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html class PeekingIterator implements Iterator<Integer> {
Iterator<Integer> ite;
boolean peekFlag = false;
Integer peekNum;

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

// Returns the next element in the iteration without advancing the iterator.
public Integer peek() {
if (!peekFlag) {
peekFlag = true;
peekNum = ite.next();
}
return peekNum;
}

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

@Override
public boolean hasNext() {
return peekFlag || ite.hasNext();
}
}</span>