0150-evaluate-reverse-polish-notation

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Description

You are given an array of strings tokens that represents an arithmetic expression in a Reverse Polish Notation.

Evaluate the expression. Return an integer that represents the value of the expression.

Note that:

• The valid operators are '+', '-', '*', and '/'.
• Each operand may be an integer or another expression.
• The division between two integers always truncates toward zero.
• There will not be any division by zero.
• The input represents a valid arithmetic expression in a reverse polish notation.
• The answer and all the intermediate calculations can be represented in a 32-bit integer.

 

Example 1:

Input: tokens = ["2","1","+","3","*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9

Example 2:

Input: tokens = ["4","13","5","/","+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6

Example 3:

Input: tokens = ["10","6","9","3","+","-11","*","/","*","17","+","5","+"]
Output: 22
Explanation: ((10 * (6 / ((9 + 3) * -11))) + 17) + 5
= ((10 * (6 / (12 * -11))) + 17) + 5
= ((10 * (6 / -132)) + 17) + 5
= ((10 * 0) + 17) + 5
= (0 + 17) + 5
= 17 + 5
= 22

 

Constraints:

• 1 <= tokens.length <= 104
• tokens[i] is either an operator: "+", "-", "*", or "/", or an integer in the range [-200, 200].

Solution(Python)

class Solution:
    # Time complexity: O(n)
    # Space complexity: O(n)
    def evalRPN(self, tokens: List[str]) -> int:
        operations = {
            "+": lambda a,b: a+b,
            "-": lambda a,b: a-b,
            "*": lambda a,b: a*b,
            "/": lambda a,b: int(a/b),
        }
        stack = []

        for token in tokens:
            if token in operations:
                num2 = stack.pop()
                num1 = stack.pop()
                operation = operations[token]
                stack.append(operation(num1, num2))
            else:
                stack.append(int(token))
        return stack.pop()