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 ;
     if (x == '*' || x == '/') return ;
     if (x == '^') return ;
     return -;
 }
 
 int infixtopostfix(string s) {
     string ans;
     stack<char> T;
     for (int i = ; 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 -;
             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 ;
 }