Friday 8 May 2020

Evaluate Reverse Polish Notation | Python

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

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

Note:

The division between two integers should truncate toward zero.
The given RPN expression is always valid. That means the expression would always evaluate to a result and there won't be any divide by zero operation.
Example 1:

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

Input: ["4", "13", "5", "/", "+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6
Example 3:

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




Code:
class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        l=[];
        for i in tokens:
            if(i=='+' or i=='-' or i=='*' or i=='/'):
                a=l.pop(-1);
                b=l.pop(-1);
                g=b+i+a;
                w=eval(g);
                e=int(w);
                l.append(str(e));
            else:
                l.append(i);
        return int(l[0]);

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