PrefixFinder

class Node {
  char c;
  Set <Node> children; 
}

class PrefixGrouper {
  ArrayList <String> result = new ArrayList<String>();
  
  public ArrayList<String> prefixWith(Node root, String prefix) {
    result.clear();
    char []p = prefix.toCharArray();
    Stack <Character> stack = new Stack<Character>();
    prefixWith(root, p, 0, stack);
    return result;
  }
  
  private ArrayList<String> prefixWith(Node node, char [] prefix, int i, Stack<Character> stack) {
    if (i < prefix.length && node != null) {
      if (node.children(contains(new Node(prefix[i])))) {
        stack.add(node.c);
        prefixWith(node, prefix, i + 1, stack);
      }
    } else if (node.children.size() == 0) {
      StringBuilder sb = new StringBuilder();
      for (char c: Collections.reverse(stack)) {
        sb.append(c);
      }
      result.append(sb.toString());
    } else {
      for (Node n: node.children) {
        stack.add(n.c);
        prefixWith(node, prefix, i, stack);
        stack.remove();
      }
    }
  }
}