Data Structure Stack: Infix to Postfix

http://geeksquiz.com/stack-set-2-infix-to-postfix/

#include <iostream>
#include <vector>
#include <algorithm>
#include <queue>
#include <stack>
#include <string>
#include <fstream>
#include <map>
#include <set>
using namespace std;

bool isoprand(char x) {
    return x >= 'A' && x <= 'Z' || x >= 'a' && x <= 'z';
}

int prec(char x) {
    if (x == '+' || x == '-') return 1;
    if (x == '*' || x == '/') return 2;
    if (x == '^') return 3;
    return -1;
}

int infixtopostfix(string s) {
    string ans;
    stack<char> T;
    for (int i = 0; i < s.size(); i++) {
        if (isoprand(s[i])) ans += s[i];
        else if (s[i] == '(') T.push(s[i]);
        else if (s[i] == ')') {
            while (!T.empty() && T.top() != '(') {
                ans += T.top();
                T.pop();
            }
            if (!T.empty() && T.top() != '(') return -1;
            T.pop();
        }
        else {
            while (!T.empty() && prec(s[i]) <= prec(T.top())) {
                ans += T.top();
                T.pop();
            }
            T.push(s[i]);
        }
    }
    while (!T.empty()) {
        ans += T.top();
        T.pop();
    }
    cout << ans << endl;
}

int main() {
    string s = "a+b*(c^d-e)^(f+g*h)-i";
    infixtopostfix(s);
    return 0;
}