evaluation_of_postfix_expression.py

"""
Given string S representing a postfix expression, the task is to evaluate the expression and find the final value. Operators will only include the basic arithmetic operators like *, /, + and -.

 

Example 1:

Input: S = "231*+9-"
Output: -4
Explanation:
After solving the given expression, 
we have -4 as result.
Example 2:

Input: S = "123+*8-"
Output: -3
Explanation:
After solving the given postfix 
expression, we have -3 as result.

Your Task:
You do not need to read input or print anything. Complete the function evaluatePostfixExpression() that takes the string S denoting the expression as input parameter and returns the evaluated value.


Expected Time Complexity: O(|S|)
Expected Auixilliary Space: O(|S|)


Constraints:
1 ≤ |S| ≤ 105

0 ≤ |Si|≤ 9 (And given operators)


"""

class Solution:
    #Function to evaluate a postfix expression.
    def evaluatePostfix(self, S):
        #code here
        stack = []

        # Iterate through the expression
        for char in S:
            # If the character is an operand, push it to the stack
            if char.isdigit():
                stack.append(int(char))
            # If the character is an operator, pop two operands from the stack,
            # perform the operation, and push the result back to the stack
            else:
                val2 = stack.pop()
                val1 = stack.pop()
                if char == '+':
                    stack.append(val1 + val2)
                elif char == '-':
                    stack.append(val1 - val2)
                elif char == '*':
                    stack.append(val1 * val2)
                elif char == '/':
                    stack.append(val1 // val2)  # Integer division

        # The final result will be the only element left in the stack
        return stack.pop()