LeetCode 3870 & 3871: Same Problem, Different Constraints Demand Different Solutions

LeetCode problems 3870 and 3871 share identical descriptions — counting commas used when writing all integers from 1 to n in standard number formatting. However, their differing constraints lead to fundamentally different solution approaches. Problem 3870 has a tighter constraint (n ≤ 1,000), making a simple linear loop practical, while 3871 allows n up to 10^18, requiring a more generalized mathematical approach. The generalized solution iterates over powers of 1,000, adding the count of numbers that gain an additional comma at each threshold. Because a new comma appears every three digits, the loop runs only a handful of times even for the largest inputs, keeping the solution effectively constant-time.