Longest Common Substring in DSA Typescript - Build from Scratch

Longest Common Substring

📖 Scenario: You are working on a text comparison tool that helps users find the longest matching part between two texts. This is useful in plagiarism detection, DNA sequence analysis, or finding common phrases.

🎯 Goal: Build a TypeScript program that finds the longest common substring between two given strings.

📋 What You'll Learn

Create two string variables with exact values

Create a variable to hold the length of the first string

Implement nested loops to compare substrings

Use a variable to track the longest common substring found

Print the longest common substring at the end

💡 Why This Matters

🌍 Real World

Finding longest common substrings is useful in text comparison tools, DNA sequence analysis, and plagiarism detection.

💼 Career

Understanding string matching algorithms is important for software developers working in data processing, bioinformatics, and search engines.

1

Create the input strings

Create two string variables called str1 and str2 with the exact values "abcdef" and "zabcf" respectively.

DSA Typescript

// Your code here

Hint

Use const to declare the strings with the exact text given.

2

Create a variable for the length of the first string

Create a variable called len1 and set it to the length of str1 using str1.length.

DSA Typescript

const str1: string = "abcdef";
const str2: string = "zabcf";
// Your code here

Hint

Use str1.length to get the length of the first string.

3

Find the longest common substring

Create a variable called longestSubstr and set it to an empty string. Use nested for loops with variables i and j to iterate over str1 and str2. Inside, use a while loop with variable k to compare characters and find matching substrings. Update longestSubstr if a longer common substring is found.

DSA Typescript

const str1: string = "abcdef";
const str2: string = "zabcf";
const len1: number = str1.length;
// Your code here

Hint

Use nested loops to check every possible substring start. Use a while loop to extend the match. Update the longest substring when a longer one is found.

4

Print the longest common substring

Use console.log to print the value of longestSubstr.

DSA Typescript

const str1: string = "abcdef";
const str2: string = "zabcf";
const len1: number = str1.length;

let longestSubstr: string = "";

for (let i = 0; i  longestSubstr.length) {
            longestSubstr = str1.substring(i, i + k);
        }
    }
}
// Your code here

Hint

Use console.log(longestSubstr) to show the result.

Practice

1. What does the Longest Common Substring problem find between two strings s1 and s2?

easy

A. The longest sequence of characters that appear in order but not necessarily contiguously

B. The total number of common characters between the two strings

C. The longest sequence of characters that appear contiguously in both strings

D. The shortest substring that contains all characters from both strings

Solution

1. Step 1: Understand substring vs subsequence

   A substring is a continuous part of a string, while a subsequence can skip characters.

2. Step 2: Define Longest Common Substring

   It finds the longest continuous sequence of characters present in both strings.

3. Final Answer:

   The longest sequence of characters that appear contiguously in both strings -> Option C

4. Quick Check:

   Longest Common Substring = continuous match [OK]

Hint: Look for continuous matching characters, not scattered ones [OK]

Common Mistakes:

• Confusing substring with subsequence
• Thinking order doesn't matter
• Counting total common characters instead

2. Which of the following TypeScript function signatures correctly represents a function to find the longest common substring length between two strings s1 and s2?

easy

A. function longestCommonSubstring(s1: string, s2: string): number

B. function longestCommonSubstring(s1: number[], s2: number[]): string

C. function longestCommonSubstring(s1: string, s2: string): string[]

D. function longestCommonSubstring(s1: string, s2: string): void

Solution

1. Step 1: Identify input types

   The inputs are two strings, so parameters should be string.

2. Step 2: Identify output type

   The function returns the length of the longest common substring, which is a number.

3. Final Answer:

   function longestCommonSubstring(s1: string, s2: string): number -> Option A

4. Quick Check:

   Input strings, output number length [OK]

Hint: Match input/output types to problem description [OK]

Common Mistakes:

• Using arrays instead of strings as input
• Returning string or array instead of number
• Using void return type mistakenly

3. What is the output of the following TypeScript code?

function longestCommonSubstring(s1: string, s2: string): number {
  const m = s1.length, n = s2.length;
  let maxLen = 0;
  const dp: number[][] = Array(m + 1).fill(0).map(() => Array(n + 1).fill(0));
  for (let i = 1; i <= m; i++) {
    for (let j = 1; j <= n; j++) {
      if (s1[i - 1] === s2[j - 1]) {
        dp[i][j] = dp[i - 1][j - 1] + 1;
        maxLen = Math.max(maxLen, dp[i][j]);
      }
    }
  }
  return maxLen;
}

console.log(longestCommonSubstring('abcde', 'abfde'));

medium

A. 2

B. 3

C. 4

D. 1

Solution

1. Step 1: Compare characters of 'abcde' and 'abfde'

   Matching continuous substrings are 'ab' at start and 'de' at end, both length 2.

2. Step 2: Find longest continuous match

   Longest common substring length is 2.

3. Final Answer:

   2 -> Option A

4. Quick Check:

   Longest common substring length = 2 [OK]

Hint: Check continuous matching parts, not scattered matches [OK]

Common Mistakes:

• Counting non-continuous matches
• Confusing longest common subsequence with substring
• Off-by-one errors in indexing

4. The following TypeScript code attempts to find the longest common substring length but has a bug. What is the bug?

function longestCommonSubstring(s1: string, s2: string): number {
  const m = s1.length, n = s2.length;
  let maxLen = 0;
  const dp: number[][] = Array(m).fill(0).map(() => Array(n).fill(0));
  for (let i = 1; i <= m; i++) {
    for (let j = 1; j <= n; j++) {
      if (s1[i - 1] === s2[j - 1]) {
        dp[i][j] = dp[i - 1][j - 1] + 1;
        maxLen = Math.max(maxLen, dp[i][j]);
      }
    }
  }
  return maxLen;
}

medium

A. Loop indices should start from 0 instead of 1

B. dp array is initialized with wrong size causing index errors

C. maxLen is not updated inside the loop

D. Comparison uses wrong characters from strings

Solution

1. Step 1: Check dp array initialization

   dp is created with size m x n but accessed with indices up to m and n (1-based), causing out-of-bound errors.

2. Step 2: Correct dp size

   dp should be (m+1) x (n+1) to safely use 1-based indexing in loops.

3. Final Answer:

   dp array is initialized with wrong size causing index errors -> Option B

4. Quick Check:

   dp size must match loop indices [OK]

Hint: Match dp size with loop ranges to avoid index errors [OK]

Common Mistakes:

• Not adding +1 to dp dimensions
• Starting loops at 1 but dp size too small
• Ignoring index out-of-bound runtime errors

5. Given two strings s1 = 'xabxac' and s2 = 'abcabxabcd', what is the length of their longest common substring?

hard

A. 3

B. 5

C. 6

D. 4

Solution

1. Step 1: Identify common substrings

   Common substrings include 'abx' and 'abxa'. 'abxa' is length 4 and appears in both strings.

2. Step 2: Confirm longest substring length

   No longer continuous substring than length 4 exists in both strings.

3. Final Answer:

   4 -> Option D

4. Quick Check:

   Longest common substring length = 4 [OK]

Hint: Look for longest continuous matching sequence visually [OK]

Common Mistakes:

• Counting non-continuous matches
• Choosing shorter common substrings
• Confusing substring with subsequence