400. Nth Digit

Description

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.

Solution

Math

很多细节需要注意！

class Solution {
    public int findNthDigit(int n) {
        int len = 1;
        int base = 1;

        while (n > 9L * base * len) {   // watch out for overflow!
            n -= 9 * base * len;
            base *= 10;
            ++len;
        }
        
        // don't forget to minus 1 because base matters
        return getDigit(base + (n - 1) / len, len - (n - 1) % len);
    }
    
    // if n = 6104 and i = 2, return 0
    public int getDigit(int n, int i) {
        while (i > 1) {
            n /= 10;
            --i;
        }
        
        return n % 10;
    }
}