0284. Peeking Iterator (M)

Peeking Iterator (M)

题目

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?

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题意

在只有next()和hasNext()方法的Iterator基础上实现PeekingIterator，扩展出一个peek()方法。

思路

在类内部加一个缓存变量，预先对传进来的Iterator进行请求。

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代码实现

Java

// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html

class PeekingIterator implements Iterator<Integer> {
    private Iterator<Integer> iterator;
    private Integer cur;

    public PeekingIterator(Iterator<Integer> iterator) {
        // initialize any member here.
        this.iterator = iterator;
        this.cur = iterator.hasNext() ? iterator.next() : null;
    }

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

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

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

JavaScript

/**
 * @param {Iterator} iterator
 */
var PeekingIterator = function (iterator) {
  this.it = iterator
  this.top = this.it.hasNext() ? this.it.next() : null
}

/**
 * @return {number}
 */
PeekingIterator.prototype.peek = function () {
  return this.top
}

/**
 * @return {number}
 */
PeekingIterator.prototype.next = function () {
  const top = this.top
  this.top = this.it.hasNext() ? this.it.next() : null
  return top
}

/**
 * @return {boolean}
 */
PeekingIterator.prototype.hasNext = function () {
  return this.top !== null
}