989. Add to Array-Form of Integer

For a non-negative integer `X`, the array-form of `X` is an array of its digits in left to right order.  For example, if `X = 1231`, then the array form is `[1,2,3,1]`.

Given the array-form `A` of a non-negative integer `X`, return the array-form of the integer `X+K`.

Example 1:

```Input: A = [1,2,0,0], K = 34
Output: [1,2,3,4]
Explanation: 1200 + 34 = 1234
```

Example 2:

```Input: A = [2,7,4], K = 181
Output: [4,5,5]
Explanation: 274 + 181 = 455
```

Example 3:

```Input: A = [2,1,5], K = 806
Output: [1,0,2,1]
Explanation: 215 + 806 = 1021
```

Example 4:

```Input: A = [9,9,9,9,9,9,9,9,9,9], K = 1
Output: [1,0,0,0,0,0,0,0,0,0,0]
Explanation: 9999999999 + 1 = 10000000000
```

Noteļ¼

1. `1 <= A.length <= 10000`
2. `0 <= A[i] <= 9`
3. `0 <= K <= 10000`
4. If `A.length > 1`, then `A[0] != 0`

989. Add to Array-Form of Integer
``````struct Solution;

impl Solution {
fn add_to_array_form(mut a: Vec<i32>, mut k: i32) -> Vec<i32> {
let mut i = a.len() - 1;
while k > 0 {
let sum = a[i] + k;
a[i] = sum % 10;
k = sum / 10;
if i > 0 {
i -= 1;
} else {
a.insert(0, 0);
}
}
if a.len() > 1 && a[0] == 0 {
a.remove(0);
}
a
}
}

#[test]
fn test() {
let a = vec![1, 2, 0, 0];
let k = 34;
let res = vec![1, 2, 3, 4];
let a = vec![2, 7, 4];
let k = 181;
let res = vec![4, 5, 5];