Basic Calculator II

Description

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, +, -, *, / operators and empty spaces . The integer division should truncate toward zero.

Example 1:

Input: "3+2*2"
Output: 7

Example 2:

Input: " 3/2 "
Output: 1

Example 3:

Input: " 3+5 / 2 "
Output: 5

Note:

• You may assume that the given expression is always valid.
• Do not use the eval built-in library function.

Solution(javascript)

/**
 * @param {string} s
 * @return {number}
 */
const calculate = function (s) {
  const stack = []
  let current = ''
  const newS = `${s} `
  for (const c of newS) {
    if (c === '*' || c === '/' || c === '+' || c === '-' || c === ' ') {
      if (current !== '') {
        stack.push(parseInt(current, 10))
        current = ''
      }
      while (Number.isInteger(stack[stack.length - 1]) && (
        stack[stack.length - 2] === '/'
        || stack[stack.length - 2] === '*'
      )) {
        const b = stack.pop()
        const operator = stack.pop()
        const a = stack.pop()
        stack.push(
          operator === '*' ? a * b : Math.floor(a / b),
        )
      }
      if (c !== ' ') {
        stack.push(c)
      }
    } else {
      current += c
    }
    // console.log(stack)
  }

  if (current !== '') {
    stack.push(parseInt(current, 10))
    current = ''
  }

  return stack.reduce((acc, item, index) => {
    if (item === '-') {
      stack[index + 1] = -stack[index + 1]
      return acc
    } if (item === '+') {
      return acc
    }
    return acc + item
  }, 0)
}