# 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
// 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;
    }
}
```
