Substring Search Patterns in DSA Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main goal of substring search in strings?

To find if a smaller string (pattern) exists inside a larger string (text), and if so, where it starts.

beginner

Explain the naive substring search approach.

Check every position in the text to see if the pattern matches starting there. It compares characters one by one until a match or mismatch is found.

intermediate

What is the key idea behind the Knuth-Morris-Pratt (KMP) algorithm?

KMP uses a precomputed table to skip unnecessary comparisons by remembering how the pattern matches itself, so it doesn't start over from scratch after a mismatch.

intermediate

What does the prefix function (also called failure function) in KMP represent?

It shows the length of the longest proper prefix of the pattern that is also a suffix ending at each position, helping to decide where to continue after a mismatch.

beginner

Why is substring search important in real life?

It helps in searching words in documents, DNA sequence matching, finding patterns in data, and many other tasks where we look for smaller parts inside bigger data.

What does the naive substring search algorithm do when it finds a mismatch?

A Moves the pattern one position to the right and starts comparing again from the first character

B Skips multiple characters based on a precomputed table

C Stops searching immediately

D Jumps to the end of the text

The naive approach moves the pattern one step right and compares from the start again.

Which algorithm uses a prefix function to optimize substring search?

A Naive Search

B Knuth-Morris-Pratt (KMP)

C Bubble Sort

D Binary Search

KMP uses the prefix function to avoid redundant comparisons.

What is the time complexity of the naive substring search in the worst case?

A O(log n)

B O(n + m)

C O(n * m)

D O(1)

Naive search can take O(n * m) time where n is text length and m is pattern length.

In KMP, what does the prefix function help to avoid?

A Rechecking characters that are already matched

B Sorting the text

C Using extra memory

D Searching the entire text

The prefix function helps skip rechecking matched characters after a mismatch.

Which of these is NOT a use case of substring search?

A Finding a word in a document

B Matching DNA sequences

C Detecting patterns in logs

D Sorting numbers

Sorting numbers is unrelated to substring search.

Describe how the naive substring search algorithm works step-by-step.

Think about how you look for a word in a book by checking each letter.

Explain the role of the prefix function in the KMP algorithm and how it improves search efficiency.

It remembers pattern parts to avoid starting over completely.

Practice

1. What is the main goal of substring search algorithms in strings?

easy

A. To sort all characters in a string alphabetically

B. To find if a smaller string exists inside a larger string

C. To reverse the entire string

D. To count the total number of vowels in a string

Solution

1. Step 1: Understand substring search purpose

   Substring search algorithms check if a smaller string (pattern) appears inside a bigger string (text).

2. Step 2: Compare options with substring search definition

   Only To find if a smaller string exists inside a larger string matches this goal. Sorting, reversing, or counting vowels are different tasks.

3. Final Answer:

   To find if a smaller string exists inside a larger string -> Option B

4. Quick Check:

   Substring search = find smaller string inside larger [OK]

Hint: Substring search means finding a smaller string inside a bigger one [OK]

Common Mistakes:

• Confusing substring search with sorting or reversing strings
• Thinking substring search counts characters
• Mixing substring search with string length calculation

2. Which of the following is the correct TypeScript syntax to check if string text contains substring pattern?

easy

A. text.includes(pattern)

B. text.contains(pattern)

C. text.has(pattern)

D. text.find(pattern)

Solution

1. Step 1: Recall TypeScript string methods

   TypeScript strings have a method called includes to check substring presence.

2. Step 2: Verify method names

   Only includes is valid. contains, has, and find are not string methods in TypeScript.

3. Final Answer:

   text.includes(pattern) -> Option A

4. Quick Check:

   Use includes() to check substring presence [OK]

Hint: Use includes() method to check substring in TypeScript [OK]

Common Mistakes:

• Using non-existent methods like contains() or has()
• Confusing includes() with find() which is for arrays
• Forgetting parentheses after method name

3. What will be the output of the following TypeScript code?

const text = "hello world";
const pattern = "wor";
console.log(text.indexOf(pattern));

medium

A. 6

B. -1

C. 3

D. 0

Solution

1. Step 1: Understand indexOf method

   indexOf returns the starting index of the substring if found, else -1.

2. Step 2: Find substring position in text

   In "hello world", substring "wor" starts at index 6 (0-based counting: h=0, e=1, l=2, l=3, o=4, space=5, w=6).

3. Final Answer:

   6 -> Option A

4. Quick Check:

   indexOf("wor") = 6 [OK]

Hint: indexOf returns start index or -1 if not found [OK]

Common Mistakes:

• Counting index starting from 1 instead of 0
• Confusing indexOf with includes (which returns boolean)
• Assuming indexOf returns substring length

4. Identify the error in the following TypeScript code snippet for substring search:

const text = "typescript";
const pattern = "script";
if (text.find(pattern)) {
  console.log("Found");
} else {
  console.log("Not Found");
}

medium

A. Using if statement without parentheses

B. Missing semicolon after console.log statements

C. Pattern variable is not declared

D. Using find() method on string which does not exist

Solution

1. Step 1: Check string methods in TypeScript

   Strings do not have a find() method. The correct method to check substring is includes() or indexOf().

2. Step 2: Verify other code parts

   Semicolons are optional in TypeScript, pattern is declared, and if statement syntax is correct.

3. Final Answer:

   Using find() method on string which does not exist -> Option D

4. Quick Check:

   Strings have includes(), not find() [OK]

Hint: Use includes() or indexOf(), not find() on strings [OK]

Common Mistakes:

• Assuming find() works on strings like arrays
• Thinking semicolons are mandatory causing errors
• Misreading if statement syntax

5. You want to find all starting indices of substring pattern in string text. Which approach correctly finds multiple occurrences in TypeScript?

hard

A. Use text.indexOf(pattern) once and return that single index

B. Use text.includes(pattern) repeatedly without changing index

C. Use a loop with indexOf(pattern, startIndex) updating startIndex after each find

D. Use text.findAll(pattern) method to get all indices

Solution

1. Step 1: Understand multiple substring search

   To find all occurrences, you must repeatedly search starting after the last found index.

2. Step 2: Analyze each option

   Use a loop with indexOf(pattern, startIndex) updating startIndex after each find uses a loop with indexOf and updates start index, which is correct. Use text.includes(pattern) repeatedly without changing index does not update index, so it loops infinitely or finds only once. Use text.indexOf(pattern) once and return that single index finds only one occurrence. Use text.findAll(pattern) method to get all indices uses a non-existent method.

3. Final Answer:

   Use a loop with indexOf(pattern, startIndex) updating startIndex after each find -> Option C

4. Quick Check:

   Loop with indexOf and updated start index finds all [OK]

Hint: Loop with indexOf and update start index to find all [OK]

Common Mistakes:

• Not updating start index causing infinite loops
• Using includes() which returns boolean, not index
• Expecting findAll() method which does not exist