【leetcode】Peeking Iterator

一、问题描述

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:

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 order of peek() before next() vs next() before peek().
4. 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?

二、问题分析

这是属于设计类的题目，从描述来看，peek操作会return 当前元素位置的下一个元素即，iterator.next()。但这里需要明确的是，如果先peek，再next，那么返回值是peek的元素，即当前的iterator已经next过了，不能再next，所以这里可以设置一个flag来表明是否执行过peek操作。其次，我们还需要明确的是peek操作后再peek，是返回的同一个吗？答案是：同一个。所以这里需要设置一个tmp变量来记录之前peek的值。最后需要注意下hasNext操作，如果执行到倒数第二个元素，此时再peek，那么iterator指向最后一个，hasNext会返回false，但这里其实应该返回true，此时与flag有关，具体的看代码。

三、Java AC 代码

class PeekingIterator implements Iterator<Integer> {
	private Iterator<Integer> iterator = null;
	private boolean flag = false;
	private Integer it = null;

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

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

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

	@Override
	public boolean hasNext() {
	    return flag || iterator.hasNext();
	}

	@Override
	public void remove() {
		iterator.remove();
	}
}