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

Recall & Review

beginner

What is the main goal of substring search algorithms?

To find the position(s) where a smaller string (pattern) appears within a larger string (text).

beginner

Explain the naive substring search approach.

It checks every possible position in the text to see if the pattern matches, moving one character at a time.

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 against itself.

intermediate

What does the prefix function (also called 'lps' array) represent in KMP?

It stores the length of the longest proper prefix of the pattern that is also a suffix for each prefix of the pattern.

intermediate

Why is KMP more efficient than the naive approach?

Because it avoids rechecking characters that have already been matched, leading to a time complexity of O(n + m) instead of O(n*m).

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

A Moves the pattern backward

B Moves the pattern one character forward in the text and starts matching from the beginning of the pattern

C Stops searching immediately

D Skips multiple characters based on a precomputed table

The naive approach moves the pattern one character forward and restarts matching from the first character.

Which data structure is essential for the KMP algorithm to avoid redundant comparisons?

A Prefix function (lps array)

B Stack

C Queue

D Hash map

The prefix function (lps array) helps KMP skip unnecessary comparisons.

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

A O(n + m)

B O(n)

C O(m)

D O(n * m)

Naive search can take O(n * m) time in the worst case, where n is text length and m is pattern length.

In KMP, what does a value of 0 in the prefix function mean for a position in the pattern?

A No proper prefix matches a suffix ending at that position

B The entire pattern matches

C The pattern is empty

D The pattern repeats fully

A 0 means no proper prefix is also a suffix for that prefix of the pattern.

Which substring search algorithm is best for very large texts and patterns?

A Bubble sort

B Naive search

C KMP algorithm

D Linear search

KMP is efficient for large texts and patterns due to its linear time complexity.

Describe how the KMP algorithm uses the prefix function to improve substring search.

Think about how KMP remembers pattern structure to skip ahead.

Explain the difference between naive substring search and KMP in terms of efficiency and approach.

Focus on how each handles mismatches.

Practice

1. What does the indexOf method in Java's String class return when searching for a substring that exists in the string?

easy

A. The starting index of the first occurrence of the substring

B. The length of the substring

C. The number of times the substring appears

D. A boolean indicating if the substring exists

Solution

1. Step 1: Understand the purpose of indexOf

   The indexOf method searches for the first occurrence of a substring inside a string and returns its position.

2. Step 2: Identify the return value when substring exists

   If the substring is found, indexOf returns the starting index (0-based) of that substring in the string.

3. Final Answer:

   The starting index of the first occurrence of the substring -> Option A

4. Quick Check:

   indexOf returns index, not count or boolean [OK]

Hint: Remember: indexOf returns position, not count or boolean [OK]

Common Mistakes:

• Thinking indexOf returns count of occurrences
• Assuming it returns true/false
• Confusing indexOf with substring length

2. Which of the following is the correct syntax to find the first occurrence of substring "cat" in string str in Java?

easy

A. int pos = str.find("cat");

B. int pos = str.search("cat");

C. int pos = str.indexOf("cat");

D. int pos = str.contains("cat");

Solution

1. Step 1: Recall Java String methods for substring search

   Java's String class uses indexOf to find substring positions.

2. Step 2: Check method names and return types

   find and search are not valid String methods; contains returns boolean, not int.

3. Final Answer:

   int pos = str.indexOf("cat"); -> Option C

4. Quick Check:

   Use indexOf for substring position [OK]

Hint: Use indexOf for position, contains for boolean check [OK]

Common Mistakes:

• Using non-existent methods like find or search
• Using contains which returns boolean, not index
• Confusing method names from other languages

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

String text = "hello world";
int pos = text.indexOf("lo");
System.out.println(pos);

medium

A. -1

B. 3

C. 4

D. 5

Solution

1. Step 1: Locate substring "lo" in "hello world"

   Counting indices: h(0), e(1), l(2), l(3), o(4). The substring "lo" starts at index 3 (l at 3, o at 4).

2. Step 2: Understand indexOf return value

   indexOf returns the starting index of the substring, which is 3 here.

3. Final Answer:

   3 -> Option B

4. Quick Check:

   Substring "lo" starts at index 3 [OK]

Hint: Count string indices from 0 to find substring start [OK]

Common Mistakes:

• Counting from 1 instead of 0
• Returning index of second character in substring
• Assuming -1 means substring found

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

String s = "banana";
int index = s.indexOf('an');
System.out.println(index);

medium

A. Using single quotes for substring instead of double quotes

B. Missing semicolon after indexOf call

C. indexOf method does not exist for String

D. Printing index before assignment

Solution

1. Step 1: Check substring argument type in indexOf

   The indexOf method expects a String argument, but single quotes denote a char.

2. Step 2: Identify correct syntax for substring

   Substring literals must be in double quotes, so 'an' should be "an".

3. Final Answer:

   Using single quotes for substring instead of double quotes -> Option A

4. Quick Check:

   Substring literals need double quotes [OK]

Hint: Use double quotes for substrings, single for chars [OK]

Common Mistakes:

• Using single quotes for multi-character substrings
• Confusing char and String types
• Ignoring compiler error messages

5. Given a string text, which code snippet correctly finds all starting indices of substring "ab" in text using indexOf?

hard

A. int index = 0;
while (index < text.length()) {
index = text.indexOf("ab");
System.out.print(index + " ");
index++;
}

B. for (int i = 0; i < text.length(); i++) {
if (text.indexOf("ab") == i) System.out.print(i + " ");
}

C. int index = text.indexOf("ab");
while (index != -1) {
System.out.print(index + " ");
index = text.indexOf("ab");
}

D. int index = 0;
while ((index = text.indexOf("ab", index)) != -1) {
System.out.print(index + " ");
index++;
}

Solution

1. Step 1: Understand how to find multiple occurrences with indexOf

   Use indexOf(substring, fromIndex) to search from a moving start index to find all occurrences.

2. Step 2: Analyze each option's logic

   int index = 0; while ((index = text.indexOf("ab", index)) != -1) { System.out.print(index + " "); index++; } correctly updates index after each found occurrence to avoid infinite loop and find all positions. Others either do not update index properly or repeatedly search from start.

3. Final Answer:

   int index = 0; while ((index = text.indexOf("ab", index)) != -1) { System.out.print(index + " "); index++; } -> Option D

4. Quick Check:

   Use indexOf with start index and increment after each find [OK]

Hint: Increment start index after each found substring to continue search [OK]

Common Mistakes:

• Not updating start index causing infinite loop
• Always searching from start index 0
• Confusing indexOf usage with for-loop index