Finding the common streak in two arrays in JavaScript

We are required to write a JavaScript function that takes in two arrays of literals, let's call them arr1 and arr2.

The function should find the longest common streak of literals in the arrays. The function should finally return an array of those literals.

For example ?

If the input arrays are ?

const arr1 = ['a', 'b', 'c', 'd', 'e'];
const arr2 = ['k', 'j', 'b', 'c', 'd', 'w'];

Then the output array should be ?

['b', 'c', 'd']

Algorithm Overview

This problem uses dynamic programming to find the Longest Common Subsequence (LCS). The approach involves:

• Converting arrays to strings for character-by-character comparison
• Building a 2D matrix to store LCS lengths
• Backtracking through the matrix to reconstruct the sequence

Implementation

const arr1 = ['a', 'b', 'c', 'd', 'e'];
const arr2 = ['k', 'j', 'b', 'c', 'd', 'w'];

const longestCommonSubsequence = (arr1 = [], arr2 = []) => {
    let str1 = arr1.join('');
    let str2 = arr2.join('');
    
    // Create 2D array for dynamic programming
    const arr = Array(str2.length + 1).fill(null).map(() => 
        Array(str1.length + 1).fill(null)
    );
    
    // Initialize first row and column with zeros
    for (let j = 0; j  0 && i > 0) {
        if (str1[j - 1] === str2[i - 1]) {
            res.unshift(str1[j - 1]);
            j -= 1;
            i -= 1;
        } else if (arr[i][j] === arr[i][j - 1]) {
            j -= 1;
        } else {
            i -= 1;
        }
    }
    
    return res;
};

console.log(longestCommonSubsequence(arr1, arr2));

Output

['b', 'c', 'd']

How It Works

The algorithm works in three phases:

1. Matrix Building: Creates a 2D matrix where each cell [i][j] represents the length of LCS between the first i characters of str2 and first j characters of str1
2. Dynamic Programming: Fills the matrix by comparing characters and taking the maximum LCS length from adjacent cells
3. Backtracking: Reconstructs the actual subsequence by tracing back through the matrix

Example with Different Arrays

const arr3 = ['x', 'y', 'z', 'a', 'b'];
const arr4 = ['a', 'b', 'x', 'y'];

console.log(longestCommonSubsequence(arr3, arr4));

['x', 'y']

Conclusion

This dynamic programming solution efficiently finds the longest common subsequence between two arrays with O(m×n) time complexity. The algorithm is particularly useful for sequence comparison and pattern matching tasks.