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Deven
60 lines
1.6 KiB
Python
60 lines
1.6 KiB
Python
class Solution:
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def calculate(self, s: str) -> int:
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"""
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Complexities:
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Time: O(n)
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Space: O(n)
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where n = len(s)
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"""
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numbers = "0123456789"
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# Read from left to right: save in stack (full number or operator)
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# - save number
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# - on operator:
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# -- if * or /, evaluate
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# -- if + or -, wait and continue
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currentNumber = 0
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stack = []
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# Evaluate numbers and create stack (of additions + subtractions)
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for c in s + "\0":
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if c == " ":
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continue
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elif c in numbers:
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digit = int(c)
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currentNumber = (currentNumber * 10) + digit
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else:
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stack.append(currentNumber)
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currentNumber = 0
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if len(stack) >= 3 and stack[-2] in "*/":
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rOperand = stack.pop()
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operator = stack.pop()
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lOperand = stack.pop()
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if operator == "*":
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stack.append(lOperand * rOperand)
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elif operator == "/":
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stack.append(int(lOperand / rOperand))
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stack.append(c)
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# Pop off null terminator
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stack.pop()
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# Evaluate all additions and subtractions from stack
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current = stack[0]
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for i in range(1, len(stack) - 1, 2):
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operator = stack[i]
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rOperand = stack[i + 1]
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if operator == "+":
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current += rOperand
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elif operator == "-":
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current -= rOperand
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return current
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