Can you find the digit?

Computer Science Level 3

$S=123456789101112131415161718192021\ldots$

Given that the string $S$ is the concatenation of the first 1,000,000,000 positive integers in ascending order, find the $999999999^{\text{th}}$ digit of $S$ .

Details and Assumptions:

By the $x^{\text{th}}$ digit of $S$ , we mean the digit in the $x^{\text{th}}$ place from the left in the decimal representation of $S$ .

Some explicit examples are :

The $5^{\text{th}}$ digit of the number $14583469$ is $3$ .
The $4^{\text{th}}$ digit of the number $143256$ is $2$ .

8 9 7 5 4 6 2 3