TRIE data structure in Kotlin

trie.kt

class Trie() {
    
    private data class Node(val children: MutableList<Node?> = MutableList(26) { null }, var leaf: Boolean = false)
    
    private val root = Node()

    /** Inserts a word into the trie. */
    fun insert(word: String) {
        var curr = root
        for(ch in word) {
            val idx = ch.idx()
            if(curr.children[idx] == null) {
                curr.children[idx] = Node()
            }
            curr = curr.children[idx]!!
        }
        curr.leaf = true
    }

    /** Returns if the word is in the trie. */
    fun search(word: String): Boolean {
        var curr = root
        for(ch in word) {
            val idx = ch.idx()
            if(curr.children[idx] == null) {
                return false
            }
            curr = curr.children[idx]!!
        }
        return curr.leaf
    }

    /** Returns if there is any word in the trie that starts with the given prefix. */
    fun startsWith(prefix: String): Boolean {
        var curr = root
        for(ch in prefix) {
            val idx = ch.idx()
            if(curr.children[idx] == null) {
                return false
            }
            curr = curr.children[idx]!!
        }
        return true
    }
    
    private fun Char.idx() = toInt() - 'a'.toInt()
}