Longest possible string built from two strings in JavaScript

We need to write a JavaScript function that takes two strings containing only letters from a to z and returns the longest possible sorted string with distinct letters from both strings combined.

Problem Statement

Given two strings s1 and s2, create a function that:

• Combines both strings
• Removes duplicate characters
• Returns a sorted string containing each unique letter only once

Solution Using Array Methods

Here's an implementation that combines the strings, removes duplicates, and sorts the result:

const str1 = "xyaabbbccccdefww";
const str2 = "xxxxyyyyabklmopq";

const longestPossible = (str1 = '', str2 = '') => {
    const combined = str1.concat(str2);
    const lower = combined.toLowerCase();
    const split = lower.split('');
    const sorted = split.sort();
    const res = [];
    
    for (const el of sorted) {
        if (!res.includes(el)) {
            res.push(el);
        }
    }
    
    return res.join('');
};

console.log(longestPossible(str1, str2));

abcdefklmopqwxy

Optimized Solution Using Set

A more efficient approach using Set to automatically handle duplicates:

const str1 = "xyaabbbccccdefww";
const str2 = "xxxxyyyyabklmopq";

const longestPossibleOptimized = (str1 = '', str2 = '') => {
    const combined = str1.concat(str2).toLowerCase();
    const uniqueChars = [...new Set(combined)];
    return uniqueChars.sort().join('');
};

console.log(longestPossibleOptimized(str1, str2));

abcdefklmopqwxy

How It Works

The algorithm follows these steps:

1. Concatenate: Combine both input strings
2. Normalize: Convert to lowercase for consistency
3. Remove duplicates: Use Set or manual checking
4. Sort: Arrange characters alphabetically
5. Join: Convert back to string format

Performance Comparison

Additional Example

// Testing with different inputs
console.log(longestPossibleOptimized("aretheyhere", "yestheyarehere"));
console.log(longestPossibleOptimized("loopingisfunbutdangerous", "lessdangerousthancoding"));

aehrsty
abcdefghilmnorstuy

Conclusion

The Set-based approach provides the most efficient and readable solution for creating the longest possible string from two input strings. It automatically handles duplicates and maintains the required alphabetical sorting.