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.

Difficulty: MEDIUM_HARD
Starter code: class Solution:
    def calculate(self, s: str) -> int:
        

Solutions:
class Solution:

	def calculate(self, s):
		if not s:
			return 0
		pre_op = '+'
		stack = [0]
		cur_num = 0
		digits = '0123456789'
		s += '#'
		for c in s:
			if c == ' ':
				continue
			if c in digits:
				cur_num = cur_num * 10 + int(c)
				continue
			if pre_op == '-':
				cur_num *= -1
			elif pre_op == '*':
				cur_num *= stack.pop()
			elif pre_op == '/':
				if cur_num == 0:
					return None
				pre_num = stack.pop()
				flag = 1 if pre_num > 0 else -1
				cur_num = abs(pre_num) // cur_num * flag
			stack.append(cur_num)
			pre_op = c
			cur_num = 0
		return sum(stack)


class Solution:

	def calculate(self, s):
		if not s:
			return 0
		length = len(s)
		stack = []
		num = 0
		sign = '+'
		for (idx, c) in enumerate(s):
			if c.isdigit():
				num = num * 10 + ord(c) - ord('0')
			if c in ['+', '-', '*', '/'] or idx == length - 1:
				if sign == '+':
					stack.append(num)
				elif sign == '-':
					stack.append(-num)
				elif sign == '*':
					stack.append(stack.pop() * num)
				elif sign == '/':
					stack.append(int(stack.pop() / num))
				sign = c
				num = 0
		return sum(stack)