LeetCode #150 Evaluate Reverse Polish Notation

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Problem

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Examples

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Input: ["2", "1", "+", "3", "*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9
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Input: ["4", "13", "5", "/", "+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6
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Input: ["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

Boundary Conditions

  • Will there be any divided by zero operation?
  • Can the given RPN expression be wrong?
    • Not matched
    • Expression includes invalid characters

Solution

Method:

Time Complexity:

Space Complexity:

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class Solution:
    def evalRPN(self, tokens):
        """
        :type tokens: List[str]
        :rtype: int
        """
        if len(tokens) == 1:
            return int(tokens.pop())

        val = 0
        stack = []
        operators = set(['+', '-', '*', '/'])
        
        for token in tokens:
            if token not in operators:
                stack.append(token)
            else:
                l2 = int(stack.pop())
                l1 = int(stack.pop())
                if token == '+':
                    val = l1 + l2
                elif token == '-':
                    val = l1 - l2
                elif token == '*':
                    val = l1 * l2
                elif token == '/':
                    val = int(l1 / l2)

                stack.append(str(val))
        
        return val

or

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class Solution:
    def evalRPN(self, tokens):
        """
        :type tokens: List[str]
        :rtype: int
        """
        stack = []
        operators = {'+': lambda x, y: x + y, \
                     '-': lambda x, y: x - y, \
                     '*': lambda x, y: x * y, \
                     '/': lambda x, y: int(x/y)}
        
        for token in tokens:
            if token not in operators:
                stack.append(token)
            else:
                l2 = int(stack.pop())
                l1 = int(stack.pop())
                val = operators[token](l1, l2)
                stack.append(str(val))
        
        return int(stack.pop())