DDSA Solutions

3870. Count Commas in Range

EasyView on LeetCode
Math

Problem Overview

Commas appear every three digits from the right.

Intuition

Commas appear every three digits from the right. Here n is at most 10^5, so numbers never reach a second comma layer. Every integer from 1000 through n contributes exactly one comma.

Algorithm

  1. 1If n is below 1000, return 0.
  2. 2Otherwise return n minus 999.
  3. 3That counts integers 1000, 1001, ..., n.

Example Walkthrough

Input: n = 1002

  1. 1.1000, 1001, and 1002 each use one comma.
  2. 2.Count is 1002 - 999 = 3.

Output: 3

Common Pitfalls

  • •Do not loop thresholds unless n can exceed 999999.
  • •Use n - 999, not n - 1000, to include 1000 itself.
  • •Return 0 for n <= 999 instead of a negative value.
  • •Part II allows huge n and needs the 1000^k threshold sum.
3870.cs
C#
// Approach: n <= 10^5, so only numbers >= 1000 get a comma and each gets
// exactly one. Count is max(0, n - 999).
// Complexity: O(1) time and O(1) extra space.
public class Solution
{
    public int CountCommas(int n)
    {
        return Math.Max(0, n - 999);
    }
}
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