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.

求无穷序列里的第n个数字
比如第11个数是数字10里的0

C++（3ms）：

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 ;
        string s = to_string(start) ;
        int res = s[(n-1)%len] - '0' ;
        return res ;
    }
};

java（7ms）：

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;
        String s = Integer.toString(start);
        return Character.getNumericValue(s.charAt((n - 1) % len));
    }
}