400.java

public class Solution {
    public int findNthDigit(int n) {
        int digitCount = 1;
        long threshold = 9 * (long)Math.pow(10, digitCount - 1) * digitCount;
        
        while (n > threshold) {
            n -= threshold;
            threshold /= digitCount;
            threshold *= 10 * (++digitCount);
        }

        final int idx = (n - 1) % digitCount;
        final int num = (int)Math.pow(10, digitCount - 1) + (n - 1) / digitCount;
        return Character.getNumericValue(String.valueOf(num).charAt(idx));
    }
}