public static String addStrings(String num1, String num2) {
if (num1 == null || num2 == null) {
return null;
}
char numChar1[] = num1.toCharArray();
char numChar2[] = num2.toCharArray();
int i = numChar1.length;
int j = numChar2.length;
int add = 0;
StringBuilder builder = new StringBuilder();
while (i > 0 || j > 0 || add > 0) {
int tem = add;
if ( i > 0) {
i --;
tem = tem + num1.charAt(i) - '0';
}
if ( j > 0) {
j--;
tem = tem + num2.charAt(j) - '0';
}
add = tem / 10;
builder.append(tem % 10);
}
return builder.reverse().toString();
}