How to check if a String is balanced?

Question

I want to test if an input String is balanced. It would be balanced if there is a matching opening and closing parenthesis, bracket or brace.

example:
{} bal         


        

Answer 1

Following is a Java code sample to detect if a string is balanced.

http://introcs.cs.princeton.edu/java/43stack/Parentheses.java.html

The idea is that -

• For each opening brace ( [ {, push it on the stack.
• For closing brace ) ] }, try to pop a matching opening brace ( [ } from stack. If you can't find a matching opening brace, then string is not balanced.
• If after processing the complete string, stack is empty then string is balanced. Otherwise string is not balanced.

Answer 2

I wrote this code to solve this problem using only an integer (or maybe a byte) variable per bracket type.

public boolean checkWithIntegers(String input) {

    int brackets = 0;

    for (char c: input.toCharArray()) {

        switch (c) {

            case '(':

                brackets++;

                break;

            case ')':

                if (brackets == 0)
                    return false;

                brackets--;

                break;

            default:

                break;
        }
    }


    return brackets == 0;
}

public static void main(String... args) {

    Borrar b = new Borrar();
    System.out.println( b.checkWithIntegers("") );
    System.out.println( b.checkWithIntegers("(") );
    System.out.println( b.checkWithIntegers(")") );
    System.out.println( b.checkWithIntegers(")(") );
    System.out.println( b.checkWithIntegers("()") );

}

OBS

1. I'm assuming that an empty string is balanced. That can be changed with just a previous check.
2. I'm using only one type of bracket example, but other types can be added quickly just adding another case switch.

Hope this help. Cheers!