1  import java.util.Scanner;

  2  import java.util.Stack;

  3  

  4  /**

  5     This program uses a stack to check whether an expression has balanced 

  6     parentheses.

  7  */

  8  public class ParenChecker

  9  {

 10     public static void main(String[] args)

 11     {

 12        Scanner in = new Scanner(System.in);

 13        System.out.print("Enter an expression with { [ ( ) ] }: ");

 14        String expression = in.nextLine();

 15        

 16        Stack<Character> parens = new Stack<Character>();

 17  

 18        for (int i = 0; i < expression.length(); i++)

 19        {

 20           char ch = expression.charAt(i);

 21           if (ch == '(' || ch == '[' || ch == '{')

 22           {

 23              parens.push(ch);

 24           }

 25           else if (ch == ')' || ch == ']' || ch == '}')

 26           {

 27              if (parens.size() == 0)

 28              {

 29                 System.out.println("Not balanced.");

 30                 return;

 31              }

 32              char open = parens.pop();

 33              if (!(open == '(' && ch == ')'

 34                    || open == '[' && ch == ']'

 35                    || open == '{' && ch == '}'))

 36              {

 37                 System.out.println("Not balanced.");

 38                 return;

 39              }

 40           }

 41        }

 42        if (parens.size() == 0) 

 43        { 

 44           System.out.print("Balanced.");

 45        }

 46        else

 47        { 

 48           System.out.print("Not balanced.");

 49        }

 50     }

 51  }