400. Nth Digit

Find the nth digit of the infinite integer sequence 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, ...

Note: n is positive and will fit within the range of a 32-bit signed integer (n < 231).

Example 1:

Input:
3

Output:
3

Example 2:

Input:
11

Output:
0

Explanation:
The 11th digit of the sequence 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, ... is a 0, which is part of the number 10.

解题思路为：

1. 找出nth digit所在数字的长度；
2. 找出nth digit所在的那个数字；
3. 找出nth digit。

注意count可能会溢出，要用long。

public class Solution {
    public int findNthDigit(int n) {
        int len = 1;
        long count = 9;
        int start = 1;
        while(n > len * count) {
            n -= len * count;
            len += 1;
            count *= 10;
            start *= 10;
        }
        start += (n-1) / len;
        return String.valueOf(start).charAt((n-1) % len) - '0';
    }
}