PeekIterator.java

package medium;

import java.util.Iterator;
/**
 * ClassName: PeekIterator.java
 * Author: chenyiAlone
 * Create Time: 2019/5/17 14:24
 * Description: No.284
 * 思路:
 *      next 用于保存 next() 用于返回 peek()
 *
 *
 * 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?
 *
 */
public class PeekIterator implements Iterator<Integer> {
    private Iterator<Integer> iter;
    private Integer next;

    public PeekIterator (Iterator<Integer> iterator) {
        // initialize any member here.
        iter = iterator;
        if (iter.hasNext())
            next = iter.next();
    }

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

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

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

    }
}