leetcode284- Peeking Iterator- 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().

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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.

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

用一个全局的Integer变量来做buffer。每次都提前准备好放在buffer里。0.初始化先准备好第一个buffer内容 1.peek方法直接返回buffer。 2.hasNext()方法直接检查buffer是不是null 3.next()方法用利用temp最后返回现在buffer的内容，中间做一下更新buffer内容的操作。

实现

// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
class PeekingIterator implements Iterator<Integer> {
    private Integer buffer;
    private Iterator<Integer> iterator;
    public PeekingIterator(Iterator<Integer> iterator) {
        // initialize any member here.
        this.iterator = iterator;
        this.buffer = null;
        if (iterator.hasNext()) {
            this.buffer = iterator.next();
        }
    }

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

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

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