227. basic calculator ii

Given a string s which represents an expression, evaluate this expression and return its value. 

The integer division should truncate toward zero.

You may assume that the given expression is always valid. All intermediate results will be in the range of [-231, 231 - 1].

Note: You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as eval().

solution

Very similar logic to 224-basic-calculator, but instead of recursing inside of brackets, we just use a stack and always compute multiplication/division when we see it. Then, the numbers left just need to be added/subtracted, to maintain BEDMAS.

def calculate(self, s: str) -> int:
	def parse_number(i):
		res = 0
		while i < len(s) and s[i].isnumeric():
			res *= 10
			res += int(s[i])
			i += 1
		return i, res
 
	def compute(x, y, op):
		if op == "*":
			return x * y
		elif op == "/":
			return int(x / y)
  
	i = 0
	prev_op = "+"
	stack = []
	while i < len(s):
		if s[i].isnumeric():
			i, val = parse_number(i)
	  
			if prev_op == "+":
				stack.append(val)
			elif prev_op == "-":
				stack.append(-val)
			else: # * or /
				stack[-1] = compute(stack[-1], val, prev_op)
	  
		elif s[i] in {"+", "-", "*", "/"}:
			prev_op = s[i]
			i += 1
		else:
			i += 1
	  
	return sum(stack)