Longest Common Prefix in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to return an empty string if the input array is empty.

DSA C#

if (strs.Length == 0) return [1];

Drag options to blanks, or click blank then click option'

Anull

B0

C""

Dstrs[0]

Attempts:

3 left

💡 Hint

Common Mistakes

Returning null instead of an empty string.

Returning the first element without checking if array is empty.

If the array is empty, the longest common prefix is an empty string "".

2fill in blank

medium

Complete the code to initialize the prefix variable with the first string in the array.

DSA C#

string prefix = [1];

Drag options to blanks, or click blank then click option'

Anull

Bstrs[0]

C""

Dstrs[1]

Attempts:

3 left

💡 Hint

Common Mistakes

Using strs[1] which may not exist if array has only one element.

Initializing prefix as empty string which is less efficient.

We start with the first string as the initial prefix to compare with others.

3fill in blank

hard

Fix the error in the while loop condition to check prefix matching correctly.

DSA C#

while (strs[i].IndexOf(prefix) != [1]) { prefix = prefix.Substring(0, prefix.Length - 1); }

Drag options to blanks, or click blank then click option'

A0

B-1

C1

Dprefix.Length

Attempts:

3 left

💡 Hint

Common Mistakes

Checking for -1 which means prefix not found anywhere, but we want prefix at start.

Using prefix.Length which is invalid for IndexOf comparison.

IndexOf returns 0 if prefix is at the start of the string. We want to loop while prefix is NOT at start.

4fill in blank

hard

Fill both blanks to complete the for loop iterating over the strings starting from second element.

DSA C#

for (int i = [1]; i < [2]; i++) { /* loop body */ }

Drag options to blanks, or click blank then click option'

A1

B0

Cstrs.Length

Dstrs.Length - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Starting loop from 0 which repeats prefix initialization string.

Using strs.Length - 1 which misses the last string.

We start from index 1 to compare with prefix and loop until the end of the array.

5fill in blank

hard

Fill the blank to return the final longest common prefix after the loop.

DSA C#

return [1];

Drag options to blanks, or click blank then click option'

Astrs[0]

Bnull

C""

Dprefix

Attempts:

3 left

💡 Hint

Common Mistakes

Returning strs[0] which may not be the common prefix after adjustments.

Returning null or empty string incorrectly.

After adjusting prefix in the loop, we return it as the longest common prefix.

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