两个字符串做减法

public String fun(String a, String b) {
    if (a.length() > b.length()) {
        return sub(a, b);
    } else if (a.length() < b.length()) {
        return "-" + sub(b, a);
    } else {
        int index = 0;
        while (index < a.length()) {
            if (a.charAt(index) - '0' > b.charAt(index) - '0') {
                return sub(a, b);
            } else if (a.charAt(index) - '0' < b.charAt(index) - '0') {
                return "-" + sub(b, a);
            }
            index++;
        }
        return "0";
    }
}

public String sub(String a, String b) {
    StringBuilder sb = new StringBuilder();
    int borrow = 0;
    for (int i = a.length() - 1, j = b.length() - 1; i >= 0 || j >= 0; i--, j--) {
        int x = i >= 0 ? a.charAt(i) - '0' : 0;
        int y = j >= 0 ? b.charAt(j) - '0' : 0;
        int z = (x - borrow - y + 10) % 10;
        sb.append(z);
        borrow = x - borrow - y < 0 ? 1 : 0;
    }
    StringBuilder res = new StringBuilder();
    String temp = sb.reverse().toString();
    for (char ch : temp.toCharArray()) {
        if (res.length() == 0 && ch == '0') {
            continue;
        }
        res.append(ch);
    }
    return res.toString();
}