postfix_expression_with_brackets.cpp

#include <iostream>
#include <stack>
#include <string>
using namespace std;

string in_to_post(string postfix);
bool isoperator(char c);
int priority(char c);

int main() {
    int t;
    cin >> t;
    while (t--) {
        string infix;
        cin >> infix;
        cout << in_to_post(infix) << endl;
    }
    return 0;
}

string in_to_post(string infix) {
    string result;
    stack<char> stk;
    for (char c : infix) {
        if (isoperator(c)) {  // In this case, '(' and ')' are not operators.
            if (stk.empty() || priority(c) < priority(stk.top())) {
                stk.push(c);
            } else {
                while (!stk.empty() && c != '(' && c != ')' && priority(c) >= priority(stk.top())) {
                    result += stk.top();
                    stk.pop();
                }
                stk.push(c);
            }
        } else if (c == '(') {
            stk.push(c);
        } else if (c == ')') {
            while (!stk.empty() && stk.top() != '(') {
                result += stk.top();
                stk.pop();
            }
            stk.pop();  // Pop the remaining '('.
        } else {
            result += c;
        }
    }
    while (!stk.empty()) {
        result += stk.top();
        stk.pop();
    }
    return result;
}
bool isoperator(char c) {
    if (string("+-*/%").find(c) != string::npos) {
        return true;
    } else {
        return false;
    }
}
/* Priority Function
This function is used to determine an operator's priority.
Note that the priority is higher when the function returns a SMALLER value.
*/
int priority(char c) {
    if (isoperator(c)) {
        switch (c) {
            case '*':
            case '/':
            case '%':
                return 3;
            case '+':
            case '-':
                return 4;
        }
    }
    return 9;
}