PrefixTree.java

class PrefixNode {
    boolean isWord = false;
    Character c = null;
    PrefixNode[] children = new PrefixNode[26];
}

PrefixNode buildTree(List<String> words) {
    PrefixNode root = new PrefixNode();
    for (String word : words) {
        PrefixNode node = root;
        for (char c : word.toCharArray()) {
            if (node.children[c] == null) {
                PrefixNode newNode = new PrefixNode();
                newNode.c = c;
                node.children[c] = newNode;
            } else {
                node = node.children[c];
            }
        }
        node.isWord = true;
    }
    
    return root;
}