LeetCode 3491: Phone Number Prefix Solution

Master LeetCode problem 3491 (Phone Number Prefix), a easy challenge, with our optimized solutions in Java, C++, and Python. Explore detailed explanations, test your code in our interactive editor, and prepare for coding interviews.

3491. Phone Number Prefix

Easy

Array String Trie Sorting

Problem Explanation

Explanation:

To solve this problem, we can iterate through the list of phone numbers and store the frequency of each unique prefix in a hashmap. We then iterate through the hashmap and check if the frequency equals the total number of phone numbers. If it does, we add the prefix to the result list. The time complexity of this solution is O(n) where n is the total number of characters in all phone numbers.

Solution Code

import java.util.*;

class Solution {
    public List<String> findPrefix(List<String> phoneNumbers) {
        Map<String, Integer> prefixFreq = new HashMap<>();
        List<String> result = new ArrayList<>();

        for (String number : phoneNumbers) {
            for (int i = 1; i <= number.length(); i++) {
                String prefix = number.substring(0, i);
                prefixFreq.put(prefix, prefixFreq.getOrDefault(prefix, 0) + 1);
            }
        }

        for (String prefix : prefixFreq.keySet()) {
            if (prefixFreq.get(prefix) == phoneNumbers.size()) {
                result.add(prefix);
            }
        }

        return result;
    }
}

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