How to Solve Longest Common Prefix on LeetCode
The Longest Common Prefix problem is a favorite string manipulation question on LeetCode, testing your knowledge of string and array operations.
Problem Statement
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:
Input: strs = ["flower","flow","flight"]
Output: "fl"
Why This Problem Matters for Interviews
This problem:
- Assesses your string scanning and manipulation skills
- Encourages thinking about both vertical and horizontal scanning
- Forces you to handle edge cases and input validation
Approaches to Longest Common Prefix
1. Vertical Scanning (Optimal)
Compare characters column-by-column across all strings.
Time Complexity: O(S), where S is the sum of all characters
Space Complexity: O(1)
def longestCommonPrefix(strs: list[str]) -> str:
if not strs:
return ""
for i in range(len(strs[0])):
c = strs[0][i]
for s in strs[1:]:
if i == len(s) or s[i] != c:
return strs[0][:i]
return strs[0]
2. Sorting + First/Last Comparison
Sort the array and compare only first and last strings.
Time Complexity: O(S log n), n = number of strings
Space Complexity: O(1)
def longestCommonPrefix(strs: list[str]) -> str:
if not strs:
return ""
strs.sort()
first, last = strs[0], strs[-1]
i = 0
while i < len(first) and i < len(last) and first[i] == last[i]:
i += 1
return first[:i]
Key Interview Talking Points
- Handling empty arrays or single-string input
- Vertical vs horizontal scanning
- Why sorting helps
- Discuss early stopping when no common prefix
Big O Complexity Recap
| Approach | Time Complexity | Space Complexity |
|---|---|---|
| Vertical Scan | O(S) | O(1) |
| Sorting | O(S log n) | O(1) |
Pro Interview Tips
- Explain edge cases (empty string, no prefix).
- Clarify expected result when only one string is given.
- Practice with test cases