Longest Common Prefix in DSA C# - Build from Scratch

Longest Common Prefix

📖 Scenario: Imagine you work at a company that manages a list of product codes. You want to find the longest starting part that all product codes share. This helps in grouping similar products quickly.

🎯 Goal: Build a program that finds the longest common prefix string among an array of strings.

📋 What You'll Learn

Create an array of strings called products with exact values

Create an integer variable minLength to store the length of the shortest string

Use a loop to find the longest common prefix among all strings in products

Print the longest common prefix string

💡 Why This Matters

🌍 Real World

Finding common prefixes helps in grouping similar items, like product codes, file paths, or DNA sequences.

💼 Career

This skill is useful for software developers working on search engines, autocomplete features, and data grouping tasks.

1

Create the product codes array

Create a string array called products with these exact values: "flower", "flow", "flight"

DSA C#

# Create the string array products with the given values
# Your code here

Hint

Use string[] products = { "flower", "flow", "flight" }; to create the array.

2

Find the shortest string length

Create an integer variable called minLength and set it to the length of the shortest string in the products array using a loop

DSA C#

string[] products = { "flower", "flow", "flight" };
// Find the shortest string length and assign it to minLength
# Your code here

Hint

Start with the first string length, then check each string to find the smallest length.

3

Find the longest common prefix

Use a for loop with variable i from 0 to minLength - 1 to compare characters at position i in all strings of products. Stop when a mismatch is found. Store the common prefix in a string variable called prefix

DSA C#

string[] products = { "flower", "flow", "flight" };
int minLength = products[0].Length;
for (int i = 1; i < products.Length; i++)
{
    if (products[i].Length < minLength)
    {
        minLength = products[i].Length;
    }
}
// Find the longest common prefix and store it in prefix
# Your code here

Hint

Compare characters at each position for all strings. Stop when a difference is found.

4

Print the longest common prefix

Write Console.WriteLine(prefix); to print the longest common prefix stored in prefix

DSA C#

string[] products = { "flower", "flow", "flight" };
int minLength = products[0].Length;
for (int i = 1; i < products.Length; i++)
{
    if (products[i].Length < minLength)
    {
        minLength = products[i].Length;
    }
}
string prefix = "";
for (int i = 0; i < minLength; i++)
{
    char currentChar = products[0][i];
    for (int j = 1; j < products.Length; j++)
    {
        if (products[j][i] != currentChar)
        {
            goto EndLoop;
        }
    }
    prefix += currentChar;
}
EndLoop: ;
// Print the prefix
# Your code here

Hint

Use Console.WriteLine(prefix); to show the result.

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