LeetCode 1399: Count Largest Group ·

Table of Contents

Meta Data
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Difficulty: easy First Attempt: 2025-04-23

Total time: 15:00.00

Intuition
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At first, I didn’t fully understand the problem…
But after reading a good explanation in the comments, I realized the goal is to group numbers based on the sum of their digits, and return the count of groups that have the largest size.

My intuition was to use a hash map to record the mapping from digit sum → group size, and a constant max_count to keep track of the largest group size.
After iterating through all integers from 1 to n, we can determine the maximum group size.
Then, we iterate through the hash map to count how many groups have this size — that will be the answer.

This solution runs in O(n) time, since we just iterate through the numbers once.

Approach
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class Solution:
    def countLargestGroup(self, n: int) -> int:
        seen = {}
        max_count = 0
        for i in range(1, n+1):
            digit_sum = sum(map(int, list(str(i))))
            count = seen.get(digit_sum, 0)
            max_count = max(max_count, count+1)
            seen[digit_sum] = count + 1

        return len([v for v in seen.values() if v == max_count])