// Nœud opérateur
    public class OperatorNode extends Node {
        String operator;
        Node left, right;

        OperatorNode(String operator, Node left, Node right) {
            this.operator = operator;
            this.left = left;
            this.right = right;
        }

        @Override
        public String toPrefix() {
            return operator + " " + left.toPrefix() + " " + right.toPrefix();
        }
    }