3870 - Count Commas in Range (Easy)

Problem Link

https://leetcode.com/problems/count-commas-in-range

Problem Statement

You are given an integer n.

Return the total number of commas used when writing all integers from [1, n] (inclusive) in standard number formatting.

In standard formatting:

• A comma is inserted after every three digits from the right.
• Numbers with fewer than 4 digits contain no commas.

Example 1:

Input: n = 1002

Output: 3

Explanation:

The numbers "1,000", "1,001", and "1,002" each contain one comma, giving a total of 3.

Example 2:

Input: n = 998

Output: 0

Explanation:

All numbers from 1 to 998 have fewer than four digits. Therefore, no commas are used.

Constraints:

• 1 <= n <= 1e5

Approach 1: Math

Since $n <= 100,000$, every number in the range contains at most one comma.

• Numbers from $1$ through $999$ contain no commas.
• Numbers from $1,000$ through $n$ contain exactly one comma.

Therefore,

• if $n >= 1,000$, the number of integers containing a comma is $n - 1000 + 1 = n - 999$.
• else, no commas are used. Combining both cases, the answer is $max(n - 999, 0)$.

Complexity Analysis:

• Time Complexity: $O(1)$
• Space Complexity: $O(1)$

class Solution:
    def countCommas(self, n: int) -> int:
        return max(n - 999, 0)