The k-beauty of an integer num is defined as the number of substrings of num when it is read as a string that meet the following conditions:
It has a length of k.
It is a divisor of num.
Given integers num and k, return the k-beauty of num.
Note:
Leading zeros are allowed.
0 is not a divisor of any value.
A substring is a contiguous sequence of characters in a string.
Example 1:
Input: num = 240, k = 2
Output: 2
Explanation: The following are the substrings of num of length k:
- "24" from "240": 24 is a divisor of 240.
- "40" from "240": 40 is a divisor of 240.
Therefore, the k-beauty is 2.
Example 2:
Input: num = 430043, k = 2
Output: 2
Explanation: The following are the substrings of num of length k:
- "43" from "430043": 43 is a divisor of 430043.
- "30" from "430043": 30 is not a divisor of 430043.
- "00" from "430043": 0 is not a divisor of 430043.
- "04" from "430043": 4 is not a divisor of 430043.
- "43" from "430043": 43 is a divisor of 430043.
Therefore, the k-beauty is 2.
Constraints:
1 <= num <= 109
1 <= k <= num.length (taking num as a string)
Solutions
Solution 1: Enumeration
We can convert $num$ to a string $s$, then enumerate all substrings of $s$ with length $k$, convert them to an integer $t$, and check if $t$ is divisible by $num$. If it is, we increment the answer.
The time complexity is $O(\log num \times k)$, and the space complexity is $O(\log num + k)$.
We can maintain a sliding window of length $k$. Initially, the window contains the lowest $k$ digits of $num$. Then, for each iteration, we move the window one digit to the right, update the number in the window, and check if the number in the window is divisible by $num$. If it is, we increment the answer.
The time complexity is $O(\log num)$, and the space complexity is $O(1)$.