#772 - Basic Calculator III

Problem Description

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, '+', '-', '*', '/' operators, and open '(' and closing parentheses ')'. The integer division should truncate toward zero.

You may assume that the given expression is always valid. All intermediate results will be in the range of [-231, 231 - 1].

Note: You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as eval().

Solution

/**
 * @param {string} s
 * @return {number}
 */
var calculate = function(s) {
  let index = 0;
  return parseExpression();

  function parseExpression() {
    let result = parseTerm();

    while (index < s.length && (s[index] === '+' || s[index] === '-')) {
      const operator = s[index++];
      const term = parseTerm();
      result = operator === '+' ? result + term : result - term;
    }

    return result;
  }

  function parseTerm() {
    let result = parseFactor();

    while (index < s.length && (s[index] === '*' || s[index] === '/')) {
      const operator = s[index++];
      const factor = parseFactor();
      if (operator === '*') {
        result *= factor;
      } else {
        result = Math.trunc(result / factor);
      }
    }

    return result;
  }

  function parseFactor() {
    skipSpaces();

    if (s[index] === '(') {
      index++;
      const result = parseExpression();
      index++;
      return result;
    }

    let result = 0;
    while (index < s.length && s[index] >= '0' && s[index] <= '9') {
      result = result * 10 + parseInt(s[index++]);
    }

    return result;
  }

  function skipSpaces() {
    while (index < s.length && s[index] === ' ') {
      index++;
    }
  }
};