C++: Getting segmentation fault when using else instead of if else

Question

This is my code to convert infix expression to postfix. In the last if else condition, if I am using else then I am getting Segmentation Fault (SIGSEGV) while if I use else if and define the if condition, then the code is working absolutely fine. I cannot figure out why is this happening. This is for the test case (a+(b*c)). Logically, I have covered a-z,A-Z,(,) previously in the if-else conditions and the code should work fine if I use else for the operators. I need help here.

#include <bits/stdc++.h>
using namespace std;

int prt(char s)
{
    if (s == '(')
        return 0;
    if (s == '+' || s == '-')
        return 1;
    if (s == '*' || s == '/')
        return 2;
    if (s == '^')
        return 3;
}

int main()
{
    // your code goes here
    int T;
    cin >> T;
    while (T--) {
        string str;
        cin >> str;
        stack<char> s;

        for (int i = 0; i <= str.length(); i++) {
            if ((str[i] >= 'a' && str[i] <= 'z') || (str[i] >= 'A' && str[i] <= 'Z')) {
                cout << str[i];
            }
            else if (str[i] == '(') {
                s.push('(');
            }
            else if (str[i] == ')') {
                while (s.top() != '(') {
                    cout << s.top();
                    s.pop();
                }
                s.pop();
            }
            else if (str[i] == '+' || str[i] == '-' || str[i] == '*' || str[i] == '/' || str[i] == '^') { //This One
                while (prt(s.top()) >= prt(str[i])) {
                    cout << s.top();
                    s.pop();
                }
                s.push(str[i]);
            }
        }

        while (!s.empty()) {
            cout << s.top();
            s.pop();
        }

        cout << endl;
    }
    return 0;
}

Also, some feedback on code writing style improvement is appreciated.