Building_normal_stacks.java

import java.io.*;
import java.util.*;

public class Main {

  public static class CustomStack {
    int[] data;
    int tos;
    //tos taken to recieve the data  tos++ and the d[tos]=value

    public CustomStack(int cap) {
      data = new int[cap];
      tos = -1;

    }

    int size() {
        return tos+1;
     
    }

    void display() {
        for(int i=tos;i>0;i--)
        {
            System.out.print(data[i]+" ");
        }

        System.out.println();
     
    }

    void push(int val) {
        if(tos==data.length-1)
        {
            System.out.println("Stack overflow");
        }
        else
        {
            tos++;
            data[tos]=val;
        }
     
    }

    int pop() {
        if(tos==-1)
        {
            System.out.println("Stack underflow.");
             return -1;
        }
        else
        {
            int val=data[tos];
            tos--;
            return -1;
        }
     
    }

    int top() {
        if(tos==-1)
        {
            System.out.println("Stack overflow");
             return -1;
        }
         else
        {
            int val=data[tos];
            return -1;
        }
    }
  }

  public static void main(String[] args) throws Exception {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    int n = Integer.parseInt(br.readLine());
    CustomStack st = new CustomStack(n);

    String str = br.readLine();
    while(str.equals("quit") == false){
      if(str.startsWith("push")){
        int val = Integer.parseInt(str.split(" ")[1]);
        st.push(val);
      } else if(str.startsWith("pop")){
        int val = st.pop();
        if(val != -1){
          System.out.println(val);
        }
      } else if(str.startsWith("top")){
        int val = st.top();
        if(val != -1){
          System.out.println(val);
        }
      } else if(str.startsWith("size")){
        System.out.println(st.size());
      } else if(str.startsWith("display")){
        st.display();
      }
      str = br.readLine();
    }
  }
}