[LeetCode 284] Peeking Iterator

[LeetCode 284] Peeking Iterator

 

Given an Iterator class interface with methods:next()And hasNext(), Design and implement a PeekingIterator that supportspeek()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].

Callnext()Gets you 1, the first element in the list.

Now you callpeek()And it returns 2, the next element. Calling next()After thatStillReturn 2.

You callnext()The final time and it returns 3, the last element. CallinghasNext()After that shoshould return false.

Hint:

1. Think of looking ahead. You want to cache the next element.
2. Is one variable sufficient? Why or why not?
3. Test your design with call orderpeek()Beforenext()Vsnext()Before peek().
4. For a clean implementation, check out Google's guava library source code.

   Follow up: How wocould you extend your design to be generic and work with all types, not just integer?

   
   

   Solution:

   Use one variable to be cached peek element, every time call next (), peek () need to update its value.

    

   // Java Iterator interface reference:// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.htmlclass PeekingIterator implements Iterator
      
        {    Integer peek = null;    private Iterator
       
         iterator;public PeekingIterator(Iterator
        
          iterator) {    // initialize any member here.    this.iterator = iterator;}    // Returns the next element in the iteration without advancing the iterator.public Integer peek() {        if(peek !=null) return peek;        else {            peek = iterator.next();        }        return peek;}// hasNext() and next() should behave the same as in the Iterator interface.// Override them if needed.@Overridepublic Integer next() {    if(peek != null) {        int temp = peek;        peek = null;        return temp;    }    return iterator.next();}@Overridepublic boolean hasNext() {    if(peek != null) return true;    else return iterator.hasNext();}}