Longest Common Prefix in DSA C# - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Longest Common Prefix (LCP) in a list of strings?

The Longest Common Prefix is the longest starting substring that is common to all strings in the list. For example, for ["flower", "flow", "flight"], the LCP is "fl".

intermediate

How does horizontal scanning find the Longest Common Prefix?

Horizontal scanning compares prefixes between strings one by one from left to right, updating the prefix each time by comparing with the next string until the shortest common prefix is found.

intermediate

What is the time complexity of the vertical scanning approach for Longest Common Prefix?

The vertical scanning approach checks characters column by column across all strings. Its time complexity is O(S), where S is the sum of all characters in all strings.

beginner

In C#, which string method helps to check if a string starts with a given prefix?

The method is StartsWith(). It returns true if the string starts with the specified prefix, otherwise false.

beginner

Why is it important to handle empty input or single string input when finding the Longest Common Prefix?

If the input list is empty, the LCP is an empty string. If there is only one string, the LCP is the string itself. Handling these cases avoids errors and ensures correct results.

What is the Longest Common Prefix of ["dog", "racecar", "car"]?

A "car"

B "d"

C "" (empty string)

D "do"

There is no common prefix among the strings, so the result is an empty string.

Which approach compares characters column by column to find the Longest Common Prefix?

A Vertical scanning

B Binary search

C Divide and conquer

D Horizontal scanning

Vertical scanning checks each character position across all strings before moving to the next position.

In C#, which method can be used to check if a string starts with a certain prefix?

A Substring()

B Contains()

C EndsWith()

D StartsWith()

StartsWith() returns true if the string begins with the specified prefix.

What should be returned if the input list of strings is empty when finding the Longest Common Prefix?

A empty string ""

B null

C throw an exception

D first string in the list

An empty string is returned to indicate no common prefix.

What is the Longest Common Prefix of ["interview", "internet", "internal"]?

A "int"

B "inter"

C "inte"

D "intern"

"inter" is the longest prefix common to all three strings.

Explain how you would find the Longest Common Prefix using horizontal scanning.

Think about comparing strings one by one and shortening the prefix.

Describe edge cases to consider when implementing Longest Common Prefix.

Consider what happens when input is minimal or has no overlap.

Practice

1. What does the Longest Common Prefix (LCP) problem ask for when given an array of strings?

easy

A. The longest starting substring common to all strings

B. The longest substring found anywhere in all strings

C. The shortest string in the array

D. The number of strings in the array

Solution

1. Step 1: Understand the problem definition

   The Longest Common Prefix problem asks for the longest substring that appears at the start of every string in the array.

2. Step 2: Compare options with the definition

   The longest starting substring common to all strings matches the definition exactly. Options A, B, and C do not describe the prefix or the problem correctly.

3. Final Answer:

   The longest starting substring common to all strings -> Option A

4. Quick Check:

   Longest Common Prefix = longest starting substring [OK]

Hint: Focus on the start of strings, not anywhere inside [OK]

Common Mistakes:

• Confusing prefix with substring anywhere
• Thinking about shortest string instead
• Counting strings instead of prefix

2. Which of the following C# method signatures is correct for a function that finds the longest common prefix of a string array?

easy

A. public void LongestCommonPrefix(string[] strs)

B. public int LongestCommonPrefix(string[] strs)

C. public string LongestCommonPrefix(string[] strs)

D. public string LongestCommonPrefix(List<string> strs)

Solution

1. Step 1: Identify the expected return type

   The longest common prefix is a string, so the method should return a string.

2. Step 2: Check parameter type and return type

   public string LongestCommonPrefix(string[] strs) returns string and takes string array, which is correct. public int LongestCommonPrefix(string[] strs) returns int (wrong type). public void LongestCommonPrefix(string[] strs) returns void (no return). public string LongestCommonPrefix(List<string> strs) uses List<string> instead of array, which is valid but not matching the question's array input.

3. Final Answer:

   public string LongestCommonPrefix(string[] strs) -> Option C

4. Quick Check:

   Return type string with string[] parameter [OK]

Hint: Return string, input string array for LCP method [OK]

Common Mistakes:

• Using wrong return type like int or void
• Using List<string> instead of string[] when array expected
• Confusing parameter types

3. What is the output of the following C# code?

string[] words = {"flower", "flow", "flight"};
string prefix = LongestCommonPrefix(words);
Console.WriteLine(prefix);

string LongestCommonPrefix(string[] strs) {
    if (strs == null || strs.Length == 0) return "";
    string prefix = strs[0];
    for (int i = 1; i < strs.Length; i++) {
        while (!strs[i].StartsWith(prefix)) {
            prefix = prefix.Substring(0, prefix.Length - 1);
            if (prefix == "") return "";
        }
    }
    return prefix;
}

medium

A. "fl"

B. "flow"

C. "f"

D. "flight"

Solution

1. Step 1: Initialize prefix and iterate

   Prefix starts as "flower". Compare with "flow": "flow" does not start with "flower", so remove last char -> "flowe", then "flow", which matches.

2. Step 2: Compare prefix with "flight"

   "flight" does not start with "flow", remove last char: "flo", no; "fl", yes. So prefix becomes "fl".

3. Final Answer:

   "fl" -> Option A

4. Quick Check:

   Common prefix of all = "fl" [OK]

Hint: Shrink prefix until all strings start with it [OK]

Common Mistakes:

• Assuming prefix is first string always
• Not shrinking prefix correctly
• Returning empty string too early

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

string LongestCommonPrefix(string[] strs) {
    if (strs == null || strs.Length == 0) return "";
    string prefix = strs[0];
    for (int i = 1; i < strs.Length; i++) {
        while (!strs[i].Contains(prefix)) {
            prefix = prefix.Substring(0, prefix.Length - 1);
            if (prefix == "") return "";
        }
    }
    return prefix;
}

medium

A. Not checking for null strings inside array

B. Using Contains instead of StartsWith causes wrong prefix check

C. Prefix is not updated inside the loop

D. Loop index starts from 0 instead of 1

Solution

1. Step 1: Analyze the prefix check method

   The code uses strs[i].Contains(prefix) to check prefix, but prefix must be at the start, so StartsWith is correct.

2. Step 2: Understand impact of Contains

   Contains allows prefix anywhere in the string, so it may accept wrong prefixes that are not at the start.

3. Final Answer:

   Using Contains instead of StartsWith causes wrong prefix check -> Option B

4. Quick Check:

   Prefix must be at start, not anywhere [OK]

Hint: Use StartsWith, not Contains, to check prefix [OK]

Common Mistakes:

• Using Contains instead of StartsWith
• Ignoring null or empty strings in array
• Incorrect loop start index

5. Given the array ["interview", "interrupt", "integrate", "integral"], what is the longest common prefix? Also, explain why the prefix ends where it does.

hard

A. "integ"

B. "inte"

C. "inter"

D. "int"

Solution

1. Step 1: Compare strings character by character

   All strings start with "int". "interview" and "interrupt" continue with "er", while "integrate" and "integral" continue with "eg".

2. Step 2: Identify where prefix breaks

   "interview" and "interrupt" have "inter", but "integrate" and "integral" have "integ", so common prefix stops at "int".

3. Final Answer:

   "int" -> Option D

4. Quick Check:

   Prefix stops where any string differs [OK]

Hint: Prefix ends at first differing character among strings [OK]

Common Mistakes:

• Assuming longer prefix without checking all strings
• Ignoring difference in later strings
• Confusing similar prefixes with actual common prefix