just think it as a digit counter

In case of overflow, Add a zero to the last.

```
digits = digits[::-1] + [0]
digits[0] += 1
for i in range(len(digits)):
if digits[i] == 10:
digits[i] = 0
digits[i+1] += 1
else:
break
if digits[-1]==1:
return digits[::-1]
else :
return digits[::-1][1:]
```