Leetcode 14 - Longest Common Prefix

02-Jul-2026

Longest prefix in a single pass!

There are couple things that I needed to realize to solve this question. First was that if one word is longer than another word, the shorter one will be the start of a shared prefix. That’s pretty trivial. The second thing was “we want to return the longest common prefix”. That means it’ll be best to start comparing with a maximal string than an empty string.

After ending the recording, I tried running with a single for-loop to check out if having the length check with the prefix reduction worked as intended. I had to scrap using the const to allow swapping values to keep prefix the shortest string in this whole comparison. Other than that, everything is the same as it was before, and it worked perfectly.

class Solution {
public:
  string longestCommonPrefix(vector<string> &strs) {
    string prefix = strs.front();
    for (auto &word : strs) {
      if (word.size() < prefix.size())
        swap(word, prefix);
      while (word.compare(0, prefix.size(), prefix) != 0) {
        prefix.pop_back();
      }
    }
    return prefix;
  }
};

Performance evaluation

The time complexity would be $O(n + k)$ where $k$ is the longest string length in the vector. I think that’ll be it? Maybe that’ll make it $O(n)$. The space complexity $O(1)$.