leetcode 284: Peeking Iterator

Peeking Iterator

Total Accepted: 2156 Total Submissions: 6859 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. CallinghasNext() after that should return false.

[思路]

设一个 peeked 的flag 同时 保存 peeked 过得值.  如果已经peeked过了, next()直接返回保存的值即可.

[CODE]

// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
class PeekingIterator implements Iterator<Integer> {
    Iterator<Integer> itr;
    boolean peeked;
    int peekVal;
    
	public PeekingIterator(Iterator<Integer> iterator) {
	    // initialize any member here.
	    this.itr = iterator;
	}

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

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

	@Override
	public boolean hasNext() {
	    return peeked || itr.hasNext();
	}
}