infix to postfix using stack

#include<bits/stdc++.h>
using namespace std;
int prec(char c)
{
    if(c == '*' || c == '/')
        return 2;
    else if(c == '+' || c == '-')
        return 1;
    else
        return -1;
}

void infixToPostfix(string s)
{
    stack<char> st;
    int l = s.length();
    string ns;
    for(int i = 0; i < l; i++)
    {

        if((s[i] >= 'a' && s[i] <= 'z')||(s[i] >= 'A' && s[i] <= 'Z'))
            ns+=s[i];
        else if(s[i]=='('||s[i]=='{'||s[i]=='[')
                    st.push(s[i]);
        else if(s[i]==')'||s[i]=='}'||s[i]==']')
        {
            while(!st.empty()&&((s[i]==')'&&st.top()!='(')||(s[i]=='}'&&st.top()!='{')||(s[i]==']'&&st.top()!='[')))
                {
                    ns+=st.top();
                    st.pop();
                }
                st.pop();
        }

        else
        {
            while(!st.empty()  && prec(s[i]) <= prec(st.top()))
            {
                char c = st.top();
                st.pop();
                ns += c;
            }
            st.push(s[i]);
        }

    }
    while(!st.empty())
    {
        char c = st.top();
        st.pop();
        ns += c;
    }

    cout << ns << endl;

}

int main()
{
    string exp = "{(a+b)*c-d}*e";
    infixToPostfix(exp);
    return 0;
}