LeetCode in Kotlin

341. Flatten Nested List Iterator

Medium

You are given a nested list of integers nestedList. Each element is either an integer or a list whose elements may also be integers or other lists. Implement an iterator to flatten it.

Implement the NestedIterator class:

• NestedIterator(List<NestedInteger> nestedList) Initializes the iterator with the nested list nestedList.
• int next() Returns the next integer in the nested list.
• boolean hasNext() Returns true if there are still some integers in the nested list and false otherwise.

Your code will be tested with the following pseudocode:

initialize iterator with nestedList
res = []
while iterator.hasNext()
    append iterator.next() to the end of res
return res 

If res matches the expected flattened list, then your code will be judged as correct.

Example 1:

Input: nestedList = [[1,1],2,[1,1]]

Output: [1,1,2,1,1]

Explanation: By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,1,2,1,1].

Example 2:

Input: nestedList = [1,[4,[6]]]

Output: [1,4,6]

Explanation: By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,4,6].

Constraints:

• 1 <= nestedList.length <= 500
• The values of the integers in the nested list is in the range [-106, 106].

Solution

import com_github_leetcode.NestedInteger

/*
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * class NestedInteger {
 *     // Constructor initializes an empty nested list.
 *     constructor()
 *
 *     // Constructor initializes a single integer.
 *     constructor(value: Int)
 *
 *     // @return true if this NestedInteger holds a single integer, rather than a nested list.
 *     fun isInteger(): Boolean
 *
 *     // @return the single integer that this NestedInteger holds, if it holds a single integer
 *     // Return null if this NestedInteger holds a nested list
 *     fun getInteger(): Int?
 *
 *     // Set this NestedInteger to hold a single integer.
 *     fun setInteger(value: Int): Unit
 *
 *     // Set this NestedInteger to hold a nested list and adds a nested integer to it.
 *     fun add(ni: NestedInteger): Unit
 *
 *     // @return the nested list that this NestedInteger holds, if it holds a nested list
 *     // Return null if this NestedInteger holds a single integer
 *     fun getList(): List<NestedInteger>?
 * }
 */
class NestedIterator(nestedList: List<NestedInteger>) {
    private var flattenList = mutableListOf<Int>()
    private var index = 0

    init {
        flatten(nestedList, flattenList)
    }

    private fun flatten(nestedList: List<NestedInteger>, flattenList: MutableList<Int>) {
        nestedList.forEach { nestedInteger ->
            if (nestedInteger.isInteger()) {
                flattenList.add(nestedInteger.getInteger()!!)
            } else {
                flatten(nestedInteger.getList()!!, flattenList)
            }
        }
    }

    fun next(): Int = flattenList[index++]

    fun hasNext(): Boolean = index < flattenList.size
}

/*
 * Your NestedIterator object will be instantiated and called as such:
 * var obj = NestedIterator(nestedList)
 * var param_1 = obj.next()
 * var param_2 = obj.hasNext()
 */

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