PostFix-Infix double digit loss

Let's say I have String "50". With the following code at the end of the post I loose the 0 of the 50 since I used .charAt(0). How can I avoid this?

Additional info :

I have String[] s, which has the numbers in it [2],[5],[50]. The problem is I cannot convert 50 to char. I can easily say,

char ch = s[i].charAt(0); 

but this would loose the 0 at the end of the 50. I need this for postfix to infix converter. My converter works great except for the double digits. Here it is so far. With this code below, I can easily convert 2+5+7. But if I write 2+50 the postfix will be 250+. It is correct but later I will create an expression tree from this and evaluate. When creating the tree, the elements of the tree becomes 2,5,0,+ not 2,50,+. So, I have come up with a solution to store my infix elements in String[] array. But I need help converting it to postfix.

public class infix2post {
    public String infix2post(String infix) {
        Stack s = new Stack();
        char ch, ch2;
        String postfix = "";
        for (int i = 0; i < infix.length(); i++) {
            ch = infix.charAt(i);
            if (ch >= 48 && ch <= 57)
                postfix += ch;
            else if (ch == '(')
                s.push(ch);
            else if (ch == ')') {
                ch2 = s.pop();
                while (ch2 != '(') {
                    postfix += ch2;
                    ch2 = s.pop();
                }
            } else {
                while (priority(ch) <= priority(s.top())) {
                    ch2 = s.pop();
                    postfix += ch2;
                }
                s.push(ch);
            }
        }
        while (!s.isEmpty()) {
            ch = s.pop();
            postfix += ch;
        }
        return postfix;
    }

    public int priority(char ch) {
        if (ch == '+' || ch == '-')
            return 1;
        else if (ch == '*' || ch == '/' || ch == '%')
            return 2;
        else
            return 0;
    }
}