Is Longest Common Prefix easy or hard?

Longest Common Prefix is categorized as an Easy problem. The core idea is straightforward string comparison, but interviewers expect candidates to reason about efficiency and avoid redundant comparisons across multiple strings.

Longest Common Prefix Python/Java solution

In Python or Java, the common implementation uses horizontal scanning. Start with the first string as the prefix and repeatedly shorten it until each string begins with that prefix using substring or startsWith checks. This approach runs in O(S) time and constant extra space.

How to solve Longest Common Prefix in O(n)?

Treat n as the total number of characters processed across all strings. Use horizontal scanning: keep a candidate prefix and shrink it whenever the current string does not start with that prefix. Since each character is examined only once, the overall complexity remains linear in the total character count.

What is the best approach for Longest Common Prefix?

Horizontal scanning is the most commonly used approach. Start with the first string as the prefix and iteratively shrink it until it matches the start of every other string. The method runs in O(S) time, where S is the total number of characters across all strings, and uses O(1) extra space.

Is Longest Common Prefix asked at Google/Amazon/Meta?

Longest Common Prefix frequently appears in coding interviews at companies like Amazon, Google, and Meta as an introductory string problem. It tests basic string manipulation, iteration patterns, and understanding of algorithmic complexity.

What data structure is used in Longest Common Prefix?

Most solutions rely only on basic string operations. A Trie can also be used by inserting all strings and walking down nodes while only one child exists. The Trie approach is helpful for learning prefix trees but is usually unnecessary for a single query.

What is the time complexity of Longest Common Prefix?

The optimal solutions run in O(S) time, where S represents the total number of characters across all input strings. Each character is compared at most once during prefix checks. Space complexity is O(1) for horizontal scanning and higher for recursive or Trie-based methods.

Description Solution Video 6:31

14. Longest Common Prefix

Easy48.6% acceptance60 companies

Write a function to find the longest common prefix string amongst an array of strings.

If there is no common prefix, return an empty string "".

Example 1:

Input: strs = ["flower","flow","flight"]
Output: "fl"

Example 2:

Input: strs = ["dog","racecar","car"]
Output: ""
Explanation: There is no common prefix among the input strings.

Constraints:

1 <= strs.length <= 200
0 <= strs[i].length <= 200
strs[i] consists of only lowercase English letters.

Asked at top 20 of 60 companies

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String Trie

6:31

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14. Longest Common Prefix

Asked at top 20 of 60 companies

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