LeetCode 400. Nth Digit

原題網址：https://leetcode.com/problems/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.

方法：分析總結規律。

public class Solution {
    public int findNthDigit(int n) {
        if (n < 10) return n;
        long len = 1;
        long pow = 1;
        long count = 0;
        while (count + 9 * pow * len < n) {
            count += 9 * pow * len;
            pow *= 10;
            len++;
        }
        long base = pow;
        long offset = n - 1 - count;
        long num = base + offset / len;
        long remainder = offset % len;
        for(int i = 1; i < len - remainder; i++) {
            num /= 10;
        }
        return (int)(num % 10);
    }
}