LeetCode:Peeking Iterator

Peeking Iterator

Total Accepted: 20942 Total
Submissions: 62056 Difficulty: Medium

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.
Hint:
Think of "looking ahead". You want to cache the next element.
Is one variable sufficient? Why or why not?
Test your design with call order of 

peek()

 before 

next()

 vs 

next()

 before 

peek()

.
For a clean implementation, check out Google's
guava library source code.
Follow up: How would you extend your design to be generic and work with all types, not just integer?
Credits:

Special thanks to @porker2008 for adding this problem and creating all test cases.

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思路：

将“下一个”数缓存起来。

java code:

// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html class PeekingIterator implements Iterator<Integer> {

private Integer next;
private Iterator<Integer> iterator;

public PeekingIterator(Iterator<Integer> iterator) {
// initialize any member here.
this.iterator = iterator;

if(iterator.hasNext()) next = iterator.next();
}

// Returns the next element in the iteration without advancing the iterator.
public Integer peek() {
return next;
}

// hasNext() and next() should behave the same as in the Iterator interface.
// Override them if needed.
@Override
public Integer next() {
Integer tmp = next;
next = iterator.hasNext() ? iterator.next() : null;
return tmp;
}

@Override
public boolean hasNext() {
return next != null;
}
}