LeetCode in Kotlin

284. Peeking Iterator

Medium

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 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?

Solution

class PeekingIterator(private val iterator: Iterator<Int>) : Iterator<Int> {
    var peekedValue: Int? = null

    fun peek(): Int =
        if (peekedValue == null) {
            peekedValue = iterator.next()
            peekedValue!!
        } else {
            peekedValue!!
        }

    override fun next(): Int =
        if (peekedValue == null) {
            iterator.next()
        } else {
            val ret = peekedValue!!
            peekedValue = null
            ret
        }

    override fun hasNext(): Boolean = peekedValue != null || iterator.hasNext()
}

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

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