## Question

Given a non-negative integer represented as a *non-empty* array of digits, plus one to the integer.

You may assume the integer do not contain any leading zero, except the number 0 itself.

The digits are stored such that the most significant digit is at the head of the list.

## Thinking

- Digit 1234 is represented as [1, 2, 3, 4].
- If add one, just need to check the value of each postion. If value is less than 9, value plus one and return. Otherwise, set value as zero and continue.
- If all digits are 9 such as 999 [9,9,9], create a new array and set first digit as 1.

## Solution

### Java

```
class Solution {
public int[] plusOne(int[] digits) {
if (digits == null || digits.length == 0) {
throw new IllegalArgumentException("Input illegal and no solution.");
}
for (int i = digits.length - 1; i >= 0; i--) {
if (digits[i] < 9) {
++digits[i];
return digits;
}
digits[i] = 0;
}
// handle all digits is 9, such as 9, 99, 999 ...
int[] digits2 = new int[digits.length + 1];
digits2[0] = 1;
return digits2;
}
}
```