227. Basic Calculator II

• 时间复杂度O(n)，空间复杂度O(n)
• Runtime: 96 ms, faster than 62.50%
• Memory Usage: 48.1 MB, less than 8.93%

类似加减乘除的计算方式，对括号里面的内容进行递归。parseInt在遇到非常小的数字的时候会出现不符合预期的异常情况，换成 |0

/**
 * @param {string} s
 * @return {number}
 */
var calculate = function(s) {
  while(s.indexOf(')') > 0) {
    let end = s.indexOf(')');
    let start = s.substring(0, end).lastIndexOf('(');
    let cur = help(s.substring(start + 1, end));
    s = s.substring(0, start) + cur + s.substring(end + 1);
  }
  return help(s);
};

var help = function(s) {
  s = s.replace(/\s/g, '');
  let sign = '+';
  let stack = [];
  let res = 0;
  let len = s.length;
  let num = 0;
  let prev = ''
  for (let i = 0; i < len; i++) {
    let start = i;
    while (/[0-9]/.test(s[i]) || /[*/+-]/.test(s[i - 1]) && s[i] === '-') {
      i++;
    }
    const num = s.substring(start, i) - '0';
    switch(sign) {
      case '+':
        stack.push(num);
        break;
      case '-':
        stack.push(-num);
        break;
      case '*':
        stack.push(stack.pop() * num);
        break;
      case '/':
        stack.push((stack.pop() / num) | 0);
        break;
    }
    sign = s[i];
  }
  while (stack.length) {
    res += stack.pop();
  }
  return res + '';
};