StackImplementationOnParenthesisChecker

package strings;

import java.util.Scanner;

class StackNode {
    private String expression;
    private StackNode below;

    public StackNode getBelow() {
        return below;
    }

    public void setBelow(StackNode below) {
        this.below = below;
    }

    public String getExpression() {
        return expression;
    }

    public void setExpression(String expression) {
        this.expression = expression;
    }
}

class Stack {

    private StackNode top;

    public void push(String element) {
        StackNode node = new StackNode();
        node.setExpression(element);
        node.setBelow(top);
        top = node;
    }

    public boolean isEmpty() {
        return top == null;
    }

    public StackNode pop() {
        StackNode result = top;
        if (top != null) {
            top = top.getBelow();
        }
        return result;
    }

    public StackNode peek() {
        return top;
    }

    /**
     * To implement a 1-based indexing in the stack
     *
     * @return
     */
    public StackNode search() {
        return null;
    }


}


public class ParenthesisChecker {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("Input expression here\n");
        String input = sc.next();
        Stack myStack = new Stack();
        for (int i = 0; i < input.length(); i++) {
            if (input.charAt(i) == '(') {
                myStack.push("(");
            } else {
                myStack.pop();
            }
        }
        System.out.println((myStack.isEmpty()) ? "Expression is valid" : "It is not a valid expression");
    }
}