0284. Peeking Iterator

https://leetcode.com/problems/peeking-iterator

Description

Design an iterator that supports the peek operation on a list in addition to the hasNext and the next operations.

Implement the PeekingIterator class:

• PeekingIterator(int[] nums) Initializes the object with the given integer array nums.

• int next() Returns the next element in the array and moves the pointer to the next element.

• bool hasNext() Returns true if there are still elements in the array.

• int peek() Returns the next element in the array without moving the pointer.

Example 1:

**Input**
["PeekingIterator", "next", "peek", "next", "next", "hasNext"]
[[[1, 2, 3]], [], [], [], [], []]
**Output**
[null, 1, 2, 2, 3, false]
**Explanation**
PeekingIterator peekingIterator = new PeekingIterator([1, 2, 3]); // [**1**,2,3]
peekingIterator.next();    // return 1, the pointer moves to the next element [1,**2**,3].
peekingIterator.peek();    // return 2, the pointer does not move [1,**2**,3].
peekingIterator.next();    // return 2, the pointer moves to the next element [1,2,**3**]
peekingIterator.next();    // return 3, the pointer moves to the next element [1,2,3]
peekingIterator.hasNext(); // return False

Constraints:

• 1 <= nums.length <= 1000

• 1 <= nums[i] <= 1000

• All the calls to next and peek are valid.

• At most 1000 calls will be made to next, hasNext, and peek.

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

ac

Don't let peek() affect hasNext() and next().

// Java Iterator interface reference:
// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
class PeekingIterator implements Iterator<Integer> {
    private Iterator<Integer> iter;
    private Integer buff;

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

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

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

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