给你一个字符串表达式 s ，请你实现一个基本计算器来计算并返回它的值。

整数除法仅保留整数部分。

使用栈

• 时间复杂度O(n)，空间复杂度O(n)
• Runtime: 88 ms, faster than 89.01%
• Memory Usage: 43.2 MB, less than 64.04%

/**
 * @param {string} s
 * @return {number}
 */
var calculate = function(s) {
  s = s.replace(/\s/g, '');
  if (!s || !s.length) return 0;
  const stack = [];
  let cur = 0;
  let operation = '+';
  const len = s.length;
  for (let i = 0; i < len; i++) {
    const char = s[i];
    if (!isNaN(char)) {
      cur = cur * 10 + (s[i] - '0');
    }
    if (isNaN(char) || i === len - 1) {
      switch (operation) {
        case '+':
          stack.push(cur);
          break;
        case '-':
          stack.push(-cur);
          break;
        case '*':
          stack.push(stack.pop() * cur);
          break;
        case '/':
          stack.push(stack.pop() / cur | 0);
      }
      operation = char;
      cur = 0;
    }
  }
  let res = 0;
  while (stack.length) {
   res += stack.pop(); 
  }
  return res;
};

优化的栈方法

• 时间复杂度O(n)，空间复杂度O(1)
• Runtime: 84 ms, faster than 94.71%
• Memory Usage: 41.3 MB, less than 93.89%

/**
 * @param {string} s
 * @return {number}
 */
var calculate = function(s) {
  s = s.replace(/\s/g, '');
  if (!s || !s.length) return 0;
  let last = 0;
  let cur = 0;
  let operation = '+';
  const len = s.length;
  let res = 0;
  for (let i = 0; i < len; i++) {
    const char = s[i];
    if (!isNaN(char)) {
      cur = cur * 10 + (s[i] - '0');
    }
    if (isNaN(char) || i === len - 1) {
      switch (operation) {
        case '+':
          res += last;
          last = cur;
          break;
        case '-':
          res += last;
          last = (-cur);
          break;
        case '*':
          last = last * cur;
          break;
        case '/':
          last = (last / cur | 0);
      }
      operation = char;
      cur = 0;
    }
  }
  res += last;
  return res;
};

另外一种

• 时间复杂度O(n)，空间复杂度O(n)
• Runtime: 92 ms, faster than 76.52%
• Memory Usage: 47.3 MB, less than 21.78%

/**
 * @param {string} s
 * @return {number}
 */
var calculate = function(s) {
  s = s.replace(/\s/g, '');
  let sign = '+';
  let stack = [];
  let res = 0;
  let len = s.length;
  let num = 0;
  for (let i = 0; i < len; i++) {
    let start = i;
    while (/[0-9]/.test(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(parseInt(stack.pop() / num));
        break;
    }
    sign = s[i];
  }
  while (stack.length) {
    res += stack.pop();
  }
  return res;
};