【特例】【找规律】剑指offer44：找到第 k 位数字

力扣
数学找规律，参考链接

Python

class Solution:
    def findKthNumber(self, k: int) -> int:
        digit, start = 1, 1 # digit: 数字位数; start: 开始值
        count = 9 # 每组数字数量
        while k > count:
            k -= count
            digit += 1
            start *= 10
            count = 9 * start * digit
        num = start + (k-1) // digit     # 所求数位在从数字start开始的第[(k−1)/digit]个数字中(start为第0个数字)
        num_s = str(num)
        return int(num_s[(k-1) % digit]) # 所求数位为数字num的第(k−1)%digit位(数字的首个数位为第0位)

Java

class Solution {
   public int findKthNumber(int k) {
        int digit = 1;
        long start = 1;
        long count = 9;
        while (k > count) {
            k -= count;
            digit += 1;
            start *= 10;
            count = digit * start * 9;
        }
        long num = start + (k - 1) / digit;
        String numStr = Long.toString(num);
        int c = numStr.charAt((k - 1) % digit) - '0';
        return c;
    }
}