`Medium`	`Codewriting` 	`300`

Given a string, output its longest prefix which contains only digits.

## Example

- For `inputString = "123aa1"`, the output should be
`longestDigitsPrefix(inputString) = "123"`.

## Input/Output

- [execution time limit] 4 seconds (go)

- [input] string inputString

    Guaranteed constraints: \
    `3 ≤ inputString.length ≤ 100`.

- [output] string

## [Go] Syntax Tips

``` go
// Prints help message to the console
// Returns a string
func helloWorld(name string) string {
    fmt.Printf("This prints to the console when you Run Tests");
    return "Hello, " + name;
}
```
