LeetCode400. Nth Digit

题目

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.

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.

答案

public int findNthDigit(int n) {
        int len = 1, start = 1;
        long count = 9;
        while (n > len * count) {
            n -= len * count;
            len++;
            count *= 10;
            start *= 10;
        }
        // (n - 1) 的理解很关键，如果是 n ，则在正好 n == len 的情况下会多出去一位
        start += (n - 1) / len;
        //这里 (n - 1) 减的那一位正好和 charAt() 从 0 开始多的那一位抵消
        return String.valueOf(start).charAt((n - 1) % len) - '0';
    }