private int printLeaf(int stepNum, String s, Node node) {
        if (Objects.nonNull(node.left) && s.charAt(stepNum) == '0')
            stepNum = printLeaf(stepNum + 1, s, node.left); 
        else if (Objects.nonNull(node.right) && s.charAt(stepNum) == '1')
            stepNum = printLeaf(stepNum + 1, s, node.right);
        
        System.out.print(node.data);
        return stepNum;
    }

void decode(String s, Node root) {
        int cur = 0;
        while (cur < s.length()) cur = printLeaf(cur, s, root);
    }