Basic Calculator II in Python

Suppose we have to implement a basic calculator to evaluate a simple expression string. The expression string will hold only non-negative integers, some operators like, +, -, *, / and empty spaces. The integer division should take the quotient part only.

So if the input is like “3+2*2”, then the output will be 7.

To solve this, we will follow these steps −

• define a stack s, i := 0, x := an empty string
• for each character j in s
  • if j is not a blank character
    • append j into x
• s := x, n := length of x
• while i
• if s[i] is ‘/’, then
  • increase i by 1
  • num := number starting from ith index, then update i as the last character of the number
  • if stack top element
• otherwise when s[i] = “*”
  • increase i by 1
  • num := number starting from ith index, then update i as the last character of the number
  • stack top := num * stack top
• otherwise when s[i] = ‘-’
  • increase i by 1
  • num := number starting from ith index, then update i as the last character of the number
  • insert –num into stack
• otherwise
  • num := number starting from ith index, then update i as the last character of the number
  • insert num into stack
  • increase i by 1

Let us see the following implementation to get better understanding −

Example

class Solution(object):
   def calculate(self, s):
      """
      :type s: str
      :rtype: int
      """
      stack = []
      i = 0
      x=""
      for j in s:
      if j !=" ":
      x+=j
      s = x
      n = len(s)
      while i

Input
"3+2*2"
Output
7