public String addStrings(String num1, String num2) {
int i = num1.length() - 1;
int j = num2.length() - 1;
int need = 0;//保存进位
StringBuilder sb = new StringBuilder();
while(i >= 0 || j >= 0 || need != 0){
int t1 = (( i >= 0) ? num1.charAt(i) - '0' : 0);
int t2 = ((j >= 0) ? num2.charAt(j) - '0' : 0);
sb.append((t1 +t2 + need) % 10);
need = (t1 + t2 + need) /10;
i--;
j--;
}
return sb.reverse().toString();
}