. Complete the function evaluate_postfix(String exp): Input: "10 23 - 5 15 + +" Retur: 7 Node.java public class Node { Object info; Node next; Node(Object info, Node next){ this.info=info; this.next=next; } } // "((1+2)" // ['(','(','1','+','2',')'] // stack : ')' -> '2' -> '1' -> '(' -> '(' -> null Stack.java public class Stack { private Node top; public Stack() { top = null; } public boolean isEmpty() { return (top == null); } public void push(Object newItem) { top = new Node(newItem, top); } public Object pop() { if (isEmpty()) { System.out.println( "Trying to pop when stack is empty"); return null; } else { Node temp = top; top = top.next; return temp.info; } } void popAll() { top = null; } public Object peek() { if (isEmpty()) { System.out.println( "Trying to peek when stack is empty"); return null; } else { return top.info; } } } Runner.java public class Runner { public static void main(String[] args) { String expression1 = "((1+2)))))+(2+2))(1/2)"; boolean res = isBalanced(expression1); System.out.println(res); } public static boolean isBalanced(String exp) { System.out.println("The string expression is " + exp); Stack st = new Stack(); char[] exp_char = exp.toCharArray(); System.out.println("The char array expression is "); for (char c : exp_char) { // if the exp_char[i] == '(' // push it on stack if (c == '(') st.push(c); // if the exp_char[i] == ')' // check if stack is empty // if stack empty: return false // else: pop from stack else if (c == ')') { if (st.isEmpty()) return false; st.pop(); } } // after loop check if stack is null // if stack is null return True; //else return false return st.isEmpty(); } }

Database System Concepts

7th Edition

ISBN:9780078022159

Author:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan

Publisher:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan

Chapter1: Introduction

Section: Chapter Questions

Problem 1PE

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Question

1. Complete the function evaluate_postfix(String exp):

Input: "10 23 - 5 15 + +"

Retur: 7

Node.java

public class Node {
Object info;
Node next;

Node(Object info, Node next){
this.info=info;
this.next=next;
}
}
// "((1+2)"
// ['(','(','1','+','2',')']

// stack : ')' -> '2' -> '1' -> '(' -> '(' -> null

Stack.java

public class Stack {

private Node top;

public Stack() {

top = null;

}

public boolean isEmpty() {

return (top == null);

}

public void push(Object newItem) {

top = new Node(newItem, top);

}

public Object pop() {

if (isEmpty()) {

System.out.println(

"Trying to pop when stack is empty");

return null;

} else {

Node temp = top;

top = top.next;

return temp.info;

}

}

void popAll() {

top = null;

}

public Object peek() {

if (isEmpty()) {

System.out.println(

"Trying to peek when stack is empty");

return null;

} else {

return top.info;

}

}
}

Runner.java

public class Runner {
public static void main(String[] args) {
String expression1 = "((1+2)))))+(2+2))(1/2)";

boolean res = isBalanced(expression1);
System.out.println(res);

}

public static boolean isBalanced(String exp) {

System.out.println("The string expression is " + exp);

Stack st = new Stack();
char[] exp_char = exp.toCharArray();

System.out.println("The char array expression is ");
for (char c : exp_char) {
// if the exp_char[i] == '('
// push it on stack
if (c == '(')
st.push(c);

// if the exp_char[i] == ')'
// check if stack is empty
// if stack empty: return false
// else: pop from stack

else if (c == ')') {
if (st.isEmpty())
return false;

st.pop();
}

}

// after loop check if stack is null
// if stack is null return True;
//else return false
return st.isEmpty();
}
}

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