Longest Common Prefix

Constraints

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

Clarifying Notes

A prefix starts at index 0 of a string.
The result must be shared by every string in strs.
If any string is empty, the answer is "".

Examples (LeetCode)

["flower","flow","flight"] -> "fl"
["dog","racecar","car"] -> ""

Approach

Use horizontal scanning:

Initialize prefix as the first string.
Iterate through each remaining word.
While the current word does not start with prefix, shrink prefix by one character from the end.
If prefix becomes empty, return "" immediately.
After processing all words, return prefix.

Why this works:

The answer must be a prefix of every word.
Shrinking only when mismatch happens preserves the longest possible valid prefix.
Early return avoids unnecessary work once no prefix is possible.

Complexity:

Time: O(S) where S is the total number of characters examined across all comparisons
Space: O(1) extra space
Note: S is not the number of strings; with n strings of average length m, this is commonly expressed as O(n * m).

Implementation

export function longestCommonPrefix(strs: string[]): string {
    let prefix = strs[0];
 
    for (let i = 1; i < strs.length; i++) {
        const word = strs[i];
 
        while (!word.startsWith(prefix)) {
            prefix = prefix.slice(0, -1);
            if (prefix === "") return "";
        }
    }
 
    return prefix;
}

Edge Cases Checklist

Single-element array (answer is that string)
Includes empty string
No shared first character
Entire shortest string is the prefix
All strings identical
Large list with small shared prefix

Test Coverage

The test suite validates:

LeetCode baseline examples
Single-element array behavior
Empty-string and no-common-prefix outcomes
Full-string and shortest-string prefix outcomes
Repeated-character and long-prefix mismatch scenarios
Deterministic larger input size and near-max length strings
Defensive behavior (input array is not mutated)

Approach

Use horizontal scanning:

Initialize prefix as the first string.

Iterate through each remaining word.

While the current word does not start with prefix, shrink prefix by one character from the end.

If prefix becomes empty, return "" immediately.

After processing all words, return prefix.

Why this works:

The answer must be a prefix of every word.

Shrinking only when mismatch happens preserves the longest possible valid prefix.

Early return avoids unnecessary work once no prefix is possible.

Complexity:

Time: O(S) where S is the total number of characters examined across all comparisons

Space: O(1) extra space

Note: S is not the number of strings; with n strings of average length m, this is commonly expressed as O(n * m).

Implementation

export function longestCommonPrefix(strs: string[]): string {
    let prefix = strs[0];
 
    for (let i = 1; i < strs.length; i++) {
        const word = strs[i];
 
        while (!word.startsWith(prefix)) {
            prefix = prefix.slice(0, -1);
            if (prefix === "") return "";
        }
    }
 
    return prefix;
}

Test Coverage

The test suite validates:

LeetCode baseline examples

Single-element array behavior

Empty-string and no-common-prefix outcomes

Full-string and shortest-string prefix outcomes

Repeated-character and long-prefix mismatch scenarios

Deterministic larger input size and near-max length strings

Defensive behavior (input array is not mutated)