[Algorithm] Check for balanced parentheses using stack

Algorithm or program to check for balanced parentheses in an expression using stack data structure.

For example:

[(1+2) * (3-4)] // true
{[()]} // true
{(}) // false
{2*3) //false

 

The idea to solve the problem is:

• opening and closing parentheses should have the same number
• last unclosed should be closed first!

 

const closing = [')', ']', '}'];
const opening = ['(', '[', '{'];
const matching = {
  0: ['(', ')'],
  1: ['[', ']'],
  2: ['{', '}']
};

const contains = ary => target => ary.includes(target);
const isOpening = contains(opening);
const isClosing = contains(closing);

function balanceParentheses (str) {
  let stack = [];
  const isEmpty = stack => stack.length === 0;
  const last = stack => stack[stack.length - 1];
  
  for (let char of str) {
    // if it is open char, push to the stack
    if (isOpening(char)) {
      stack.push(char);
    } 
    // if it is closing char
    else if (isClosing(char)) {
      // if stack is not empty
      if (!isEmpty(stack)) {
        // check last element should be the pair of closing element
        const indx = closing.indexOf(char);
        const [open, close] = matching[indx];
        // if it is, then pop the last element
        if (last(stack) === open) {
          stack.pop();
        } else {
          // otherwise, return false 
          return false;
        }
      } else {
        return false;
      }
    }
  }
  
  return isEmpty(stack);
}

console.log(balanceParentheses('{[()(){}]}')); // true
console.log(balanceParentheses(')(')); // false
console.log(balanceParentheses('({)}')); // false
console.log(balanceParentheses('{2*3)')); // false
console.log(balanceParentheses('[(1+2)*3-(1-9)]')); //true