How do you solve Count Commas in Range II on LeetCode?

Count Commas in Range II (LeetCode #3871) can be solved using Brute Force or Optimal Solution. The optimal approach is Optimal Solution with O(log n) time complexity and O(1) space complexity. Key insight: Commas appear only in numbers with 4 or more digits.

What is the time complexity of Count Commas in Range II?

The optimal solution for Count Commas in Range II runs in O(log n) time and uses O(1) space. The complexity is O(log n) because we only iterate through the number of digits in n, which is logarithmic in relation to n.

What is the brute force solution for Count Commas in Range II?

The brute force approach for Count Commas in Range II is Brute Force, with O(n²) time complexity and O(1) space complexity. This approach counts commas by formatting each number from 1 to n and counting the commas. It's straightforward but inefficient for large n. The optimal Optimal Solution approach improves this to O(log n).

What algorithm or data structure is used to solve Count Commas in Range II?

Count Commas in Range II uses the following concepts: Math. The recommended patterns to study are: Mathematical Counting, Range Queries.

What are the interview tips for Count Commas in Range II?

Explain your thought process clearly. Use examples to illustrate your approach. Be prepared to discuss time and space complexity.

What are common mistakes when solving Count Commas in Range II?

Counting commas in numbers with fewer than 4 digits. Incorrectly calculating the range limits.

#3871

Count Commas in Range II

Medium

MathMathematical CountingRange Queries

LeetCode ↗

Approaches

Brute ForceOptimal

Complexity Comparison

	Brute Force	Optimal Solution★
Time	O(n²)	O(log n)
Space	O(1)	O(1)

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Intuition

Time O(log n)Space O(1)

Count the number of integers in ranges based on digit length and multiply by the number of commas each range contributes. This avoids iterating through each number.

⚙️

Algorithm

3 steps

1Step 1: Initialize a total comma counter.
2Step 2: For each range of digit lengths (1-3, 4-6, ...), calculate how many numbers are in that range up to n.
3Step 3: Multiply the count of numbers in each range by the number of commas that range contributes and add to the total.

solution.py6 lines

1def count_commas(n):
2    total_commas = 0
3    for digits in range(4, len(str(n)) + 1):
4        total_commas += (min(n, 10**digits - 1) - 10**(digits - 1) + 1) * (digits // 3 - 1)
5    return total_commas
6

ℹ

Complexity note: The complexity is O(log n) because we only iterate through the number of digits in n, which is logarithmic in relation to n.

1Commas appear only in numbers with 4 or more digits.
2Count ranges of numbers based on digit lengths.

Solutions and explanations are original Tejav content. Problem titles © LeetCode — use the LeetCode button above for the full problem statement.