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().

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?

难度: medium

题目：给定一迭代器包含方法next()和hasNext(),设计PeekIterator,支持peek()操作其返回值为下一次调用next时的值。

思路：预先设置一变量存储peek值。

Runtime: 48 ms, faster than 100.00% of Java online submissions for Peeking Iterator.
Memory Usage: 36.8 MB, less than 74.37% of Java online submissions for Peeking Iterator.

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

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

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

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