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).

class Solution {
public:
    int findNthDigit(int n) {
        long len = 1, start = 1;
        for(; n > ((start<<3) + start) * len; start = (start<<3) + (start<<1), len++) {
            n -= (((start<<3) + start) * len);
        }
        n--;
        return to_string(start + n / len)[n % len] & 0b1111;
    }
};