Longest Common Prefix in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Longest Common Prefix Function

What is the output of the following C# code that finds the longest common prefix among an array of strings?

DSA C#

using System;

class Program {
    static 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].IndexOf(prefix) != 0) {
                prefix = prefix.Substring(0, prefix.Length - 1);
                if (prefix == "") return "";
            }
        }
        return prefix;
    }

    static void Main() {
        string[] input = {"flower", "flow", "flight"};
        Console.WriteLine(LongestCommonPrefix(input));
    }
}

A "fl"

B ""

C "f"

D "flow"

💡 Hint

Check the common starting letters of all strings.

The longest common prefix among "flower", "flow", and "flight" is "fl" because all three strings start with these two letters.

❓ Predict Output

intermediate

Longest Common Prefix with No Common Letters

What will be printed by this C# program that finds the longest common prefix?

DSA C#

using System;

class Program {
    static 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].IndexOf(prefix) != 0) {
                prefix = prefix.Substring(0, prefix.Length - 1);
                if (prefix == "") return "";
            }
        }
        return prefix;
    }

    static void Main() {
        string[] input = {"dog", "racecar", "car"};
        Console.WriteLine(LongestCommonPrefix(input));
    }
}

A ""

B "d"

C "dog"

D "c"

💡 Hint

Check if any letters are common at the start of all strings.

There is no common prefix among "dog", "racecar", and "car" so the function returns an empty string.

🧠 Conceptual

advanced

Understanding the Algorithm's Loop Behavior

In the longest common prefix algorithm, why does the inner while loop reduce the prefix length until it matches the start of the current string?

A To find the shortest string in the array.

B To count the total number of characters in all strings.

C To ensure the prefix is common to all strings processed so far.

D To sort the strings alphabetically.

💡 Hint

Think about what the prefix must represent after each iteration.

The prefix must be common to all strings processed so far, so it is shortened until it matches the start of the current string.

❓ Predict Output

advanced

Output of Longest Common Prefix with Single String

What is the output of this C# program when the input array has only one string?

DSA C#

using System;

class Program {
    static 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].IndexOf(prefix) != 0) {
                prefix = prefix.Substring(0, prefix.Length - 1);
                if (prefix == "") return "";
            }
        }
        return prefix;
    }

    static void Main() {
        string[] input = {"single"};
        Console.WriteLine(LongestCommonPrefix(input));
    }
}

A "s"

B "single"

C ""

D "sing"

💡 Hint

With only one string, the prefix is the string itself.

If there is only one string, the longest common prefix is the string itself.

🧠 Conceptual

expert

Time Complexity of Longest Common Prefix Algorithm

What is the time complexity of the longest common prefix algorithm that compares strings by trimming the prefix until it matches all strings?

A O(N * log M), where N is the number of strings and M is the length of the shortest string

B O(N + M), where N is the number of strings and M is the length of the longest string

C O(M^N), where N is the number of strings and M is the length of the shortest string

D O(N * M), where N is the number of strings and M is the length of the shortest string

💡 Hint

Consider how many characters are compared in total across all strings.

The algorithm compares each character of the prefix with each string, leading to O(N * M) complexity.

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