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basic-calculator.py
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basic-calculator.py
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# Time: O(n)
# Space: O(n)
#
# Implement a basic calculator to evaluate a simple expression string.
#
# The expression string may contain open ( and closing parentheses ),
# the plus + or minus sign -, non-negative integers and empty spaces .
#
# You may assume that the given expression is always valid.
#
# Some examples:
# "1 + 1" = 2
# " 2-1 + 2 " = 3
# "(1+(4+5+2)-3)+(6+8)" = 23
#
try:
xrange # Python 2
except NameError:
xrange = range # Python 3
class Solution:
# @param {string} s
# @return {integer}
def calculate(self, s):
operands, operators = [], []
operand = ""
for i in reversed(xrange(len(s))):
if s[i].isdigit():
operand += s[i]
if i == 0 or not s[i-1].isdigit():
operands.append(int(operand[::-1]))
operand = ""
elif s[i] == ')' or s[i] == '+' or s[i] == '-':
operators.append(s[i])
elif s[i] == '(':
while operators[-1] != ')':
self.compute(operands, operators)
operators.pop()
while operators:
self.compute(operands, operators)
return operands[-1]
def compute(self, operands, operators):
left, right = operands.pop(), operands.pop()
op = operators.pop()
if op == '+':
operands.append(left + right)
elif op == '-':
operands.append(left - right)