Leetcode 284 - Peeking iterator

Note:

  • Use a this.peek to store the next();
  • Remember to reset it back to null after returning a peek.

Question:

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

Implement the PeekingIterator class:

  • PeekingIterator(Iterator<int> nums) Initializes the object with the given integer iterator iterator.
  • int next() Returns the next element in the array and moves the pointer to the next element.
  • boolean hasNext() Returns true if there are still elements in the array.
  • int peek() Returns the next element in the array without moving the pointer.

Note: Each language may have a different implementation of the constructor and Iterator, but they all support the int next() and boolean hasNext() functions.

Example:

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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

Code:

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/**
* // This is the Iterator's API interface.
* // You should not implement it, or speculate about its implementation.
* function Iterator() {
* @ return {number}
* this.next = function() { // return the next number of the iterator
* ...
* };
*
* @return {boolean}
* this.hasNext = function() { // return true if it still has numbers
* ...
* };
* };
*/

/**
* @param {Iterator} iterator
*/
var PeekingIterator = function(iterator) {
this.peeks = null;
this.iterator = iterator;
};

/**
* @return {number}
*/
PeekingIterator.prototype.peek = function() {
if (this.peeks !== null) return this.peeks;
this.peeks = this.iterator.next();
return this.peeks;
};

/**
* @return {number}
*/
PeekingIterator.prototype.next = function() {
if (this.peeks !== null) {
const peek = this.peeks;
this.peeks = null;
return peek;
}
return this.iterator.next();
};

/**
* @return {boolean}
*/
PeekingIterator.prototype.hasNext = function() {
return this.iterator.hasNext() || this.peeks !== null;
};

/**
* Your PeekingIterator object will be instantiated and called as such:
* var obj = new PeekingIterator(arr)
* var param_1 = obj.peek()
* var param_2 = obj.next()
* var param_3 = obj.hasNext()
*/