Basic Calculator

227. Basic Calculator II

A naive implementation.

For example, if we have ‘3+2*2/2+4’

• put numbers and operation into a list, which will be [3, ’+’, 2, ’*’ , 2, ’/’, 2, ’+’, 4]
• Evaluate ‘*’ and ‘/’ the first round: [3, ’+’, 2, ‘+’, 4]
• Evaluate ‘+’, ‘-’ the second round.

class Solution:
    def calculate(self, s: str) -> int:
        s = s.replace(" ","")
        nums = []
        num, operation="",""
        for c in s:
            if c.isdigit():
                num+=c
            else:
                if num:
                    nums.append(int(num))
                    num=""
                nums.append(c)
        nums.append(int(num))
        #print(nums)
        i = 0
        # 1) evaluate the '*' and '/' firstly
        while i<len(nums):
            c = nums[i]
            if c=='*':
                res = nums[i-1]*nums[i+1]
                #print("res", res)
                nums = nums[:i-1]+[res]+nums[i+2:]
                #print("nums",nums)
            elif c=='/':
                res = nums[i-1]//nums[i+1]
                nums = nums[:i-1]+[res]+nums[i+2:]
            else:
                i+=1
        res = nums[0]
        # 2) evaluate the '+' and '-'
        for i, c in enumerate(nums):
            if c=='+':
                res +=nums[i+1]
            elif c=='-':
                res -= nums[i+1]
        
        return res

A concise code from here

class Solution:
    def calculate(self, s: str) -> int:
        s = s.replace(" ","")
        num, stack, operation = 0, [], "+"
        for i in range(len(s)):
            if s[i].isdigit():
                num = num * 10 + int(s[i])
            if s[i] in "+-*/" or i == len(s) - 1:
                if operation == "+":
                    stack.append(num)
                elif operation == "-":
                    stack.append(-num)
                elif operation == "*":
                    stack.append(stack.pop()*num)
                else:
                    stack.append(int(stack.pop()/num))
                num = 0
                operation = s[i]
        return sum(stack)

Coding tips negative number // can surprise you.

>>> -3//2
-2
>>> -3/2
-1.5
>>> int(-3/2)
-1