Traverses tree level by level iteratively

IterativeTreeLevelOrderTraversal.kt

/**
 * Example:
 * var ti = TreeNode(5)
 * var v = ti.`val`
 * Definition for a binary tree node.
 * class TreeNode(var `val`: Int) {
 *     var left: TreeNode? = null
 *     var right: TreeNode? = null
 * }
 */
class Solution {
    fun levelOrder(root: TreeNode?): List<List<Int>> {
        val mutableNestedList = mutableListOf<MutableList<Int>>()

        if (root == null) return emptyList<List<Int>>()
        
        val q = mutableListOf<TreeNode?>()
        // Root node to the Queue
        q.add(root)
        while (q.isNotEmpty()) {
            // For the level out out in Int
            val level = mutableListOf<Int>()
            // add all child nodes before exhausing entire Queue 
            val childNodes = mutableListOf<TreeNode>()
            while (q.isNotEmpty()) {
                val node = q.removeFirst()
                // Collection values for the current level for the output
                node?.let {level.add(it.`val`)}
                // Collect all children nodes for the next level
                node?.left?.let {childNodes.add(it)}
                node?.right?.let {childNodes.add(it)}
            }
            // add all nodes for the next level before the next level iteration
            q.addAll(childNodes)
            // Add to the final out put list
            mutableNestedList.add(level)
        }
        

        return mutableNestedList
    }
}