public Puzzle solve_r(Stack<Puzzle> stack) {
while (!stack.empty()) {
Puzzle current = stack.pop();
if (current.isSolved()) {
return current;
}
int hashCode = current.hashCode();
if (current.getNumMoves() < maxDepth
&& (!visited.containsKey(hashCode)
|| (visited.containsKey(hashCode)
&& visited.get(hashCode) > current.getNumMoves()))) {
visited.put(hashCode, current.getNumMoves());
for (Puzzle.Move move : current.getPossibleMoves()) {
stack.push(current.swap(move));
}
}
}
return null;
}
