Traverses and collects node values in a BFS Tree Traversal

BFSRecursiveTraversal.kt

class Solution {
    fun levelOrder(root: TreeNode?): List<List<Int>> {
        val twoDList = mutableListOf<MutableList<Int>>()
        levelOrderRecursive(root, twoDList, 0)
        return twoDList
    }

    fun levelOrderRecursive(root: TreeNode?, list: MutableList<MutableList<Int>>, level: Int) {
        if (root == null) return
        
        // Since we might not have a list existing at that level, lets add if needed
        if (list.size == level) list.add(mutableListOf<Int>())
        
        list[level].add(root.`val`)
        levelOrderRecursive(root.left, list, level + 1)
        levelOrderRecursive(root.right, list, level + 1)
    }
}