After understanding that `a ^ b`

stands for the sum of each digit (ignoring carry over) and `a & b`

stands for whether carry over occurs on each digit, you will easily see that you need to add carry over `(a & b) << 1`

repeatedly until no carry over occurred. (Because first carry over may product another carry over, we should do this until no more carry over occurs)

```
func getSum(a int, b int) int {
sum, co := a^b, a&b
for co != 0 {
co = co << 1
sum, co = sum^co, sum&co
}
return sum
}
```