两个字符串做加法

public String add(String s1, String s2) {
    int m = s1.length();
    int n = s2.length();
    int carry = 0;
    int res = 0;
    StringBuilder sb = new StringBuilder();
    for (int i = m - 1, j = n - 1; i >= 0 || j >= 0; i--, j--) {
        int sum = carry;
        sum += i >= 0 ? s1.charAt(i) - '0' : 0;
        sum += j >= 0 ? s2.charAt(j) - '0' : 0;
        sb.append(sum % 10);
        carry = sum / 10;
    }
    if (carry == 1) {
        sb.append("1");
    }
    return sb.reverse().toString();
}