Postfix Notation Evaluation - Amazon Top Interview Questions

Problem Statement :

Postfix notation is a way to represent an expression where the operator comes after the operands.

For example, ["2", "2", "+", "6", "*"] would be equal to 24, since we have (2 + 2) * 6 = 24.

Given a list of strings exp, representing a postfix notation consisting of integers and operators ("+", "-", "*", "/"), evaluate the expression. "/" is integer division.

Example 1


exp = ["9", "3", "+", "2", "/"]




(9 + 3) / 2 = 6

Example 2


exp = ["3", "9", "-", "4", "/"]




(3 - 9) / 4 = -1

Solution :


                        Solution in C++ :

#define long long long
long cal(long first, string& op, long second) {
    if (op == "*") return first * second;
    if (op == "-") return first - second;
    if (op == "+") return first + second;
    return first / second;

int solve(vector<string>& exp) {
    stack<long> st;
    for (int i = 0; i < exp.size(); i++) {
        if (exp[i] == "+" or exp[i] == "-" or exp[i] == "/" or exp[i] == "*") {
            long a =;

            long b =;

            long res = cal(b, exp[i], a);


        long d = stol(exp[i]);


                        Solution in Python : 
OPERATIONS = {"+": add, "-": sub, "*": mul, "/": lambda a, b: int(a / b)}

class Solution:
    def solve(self, exp):
        stack = []

        for x in exp:
            if x in OPERATIONS:
                b, a = stack.pop(), stack.pop()
                stack.append(OPERATIONS[x](a, b))

        return stack[-1]

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