valid_braces.js

/*
 * Description:
 * validBraces takes a string of braces,
 * and determines if the order of the braces is valid.
 * validBraces should return true if the string is valid,
 * and false if it's invalid.
 * 
 * Test Cases:
 * validBraces( "(){}[]" ) => returns true 
 * validBraces( "(}" ) => returns false 
 * validBraces( "[(])" ) => returns false 
 * validBraces( "([{}])" ) => returns true 
*/

const BRACES = {
  '{': '}',
  '[': ']',
  '(': ')',
};

function validBraces(braces) {
  const stack = [];
  let isValid = true;

  for (let i = 0, len = braces.length; i < len; i++) {
    const brace = braces[i];

    // if it's an opening brace
    if (BRACES[brace]) {
      stack.push(brace);
    } else { // if it's a closing brace
      const lastOpeningBrace = stack.pop();
      // this string is not valid
      // when current brace is a closing brace
      // but not for the last opening brace
      if (BRACES[lastOpeningBrace] !== brace) {
        isValid = false;
        break;
      }
    }
  }

  return isValid;
}