public Node getPast() {
        if ( this.getRight() == null ) {
            if (this.getLeft() == null){
                return null;
            } else {
                return this.getLeft();
            }
        } else if ( this.getLeft() == null ) {
            if (this.getRight() == null){
                return null;
            } else {
                return this.getRight();
            }
        } else {
            return this.getLeft();
        }
    }