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 vector is empty.

DSA C++

if (strs.empty()) return [1];

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

A0

Bnullptr

C""

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Returning NULL or 0 instead of an empty string.

Not checking if the vector is empty before processing.

If the input vector is empty, the longest common prefix is an empty string, so we return "".

2fill in blank

medium

Complete the code to initialize the prefix with the first string in the vector.

DSA C++

std::string prefix = strs[1];

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

Afront()

B(0)

C.at(0)

D[0]

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of square brackets.

Using front() without including the correct header or context.

We use strs[0] to get the first string from the vector to start the prefix.

3fill in blank

hard

Fix the error in the while loop condition to check prefix length and character match.

DSA C++

while (prefix.length() > 0 && strs[i][[1]] == prefix[[1]]) { prefix = prefix.substr(0, prefix.length() - 1); }

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

Ai

Bprefix.length() - 1

Cprefix.length()

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using prefix.length() as index causes out-of-range error.

Using 0 as index compares only the first character.

We compare characters at index prefix.length() - 1 to check the last character of the prefix.

4fill in blank

hard

Fill both blanks to correctly iterate over the strings and update the prefix.

DSA C++

for (int [1] = 1; [1] < strs.size(); [1]++) {
    int [2] = 0;
    while ([2] < prefix.length() && strs[[1]][[2]] == prefix[[2]]) {
        [2]++;
    }
    prefix = prefix.substr(0, [2]);
}

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

Ai

Bj

Ck

Dindex

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same variable name for both loops.

Using variables that are not declared.

We use 'i' to iterate over strings and 'j' to iterate characters in prefix comparison.

5fill in blank

hard

Fill all three blanks to complete the function that finds the longest common prefix.

DSA C++

std::string longestCommonPrefix(std::vector<std::string>& strs) {
    if (strs.empty()) return [1];
    std::string prefix = strs[2];
    for (int i = 1; i < strs.size(); i++) {
        int j = [3];
        while (j < prefix.length() && strs[i][j] == prefix[j]) {
            j++;
        }
        prefix = prefix.substr(0, j);
        if (prefix.empty()) break;
    }
    return prefix;
}

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

A""

B[0]

C0

Dnullptr

Attempts:

3 left

💡 Hint

Common Mistakes

Returning nullptr instead of empty string.

Using parentheses instead of square brackets for indexing.

Not initializing j to 0 before the while loop.

Return empty string if empty input, initialize prefix with first string using [0], and use 0 to start index j.

Practice

1. What does the Longest Common Prefix (LCP) problem ask you to find given a list of strings?

easy

A. The total count of common characters in all strings

B. The longest starting substring common to all strings

C. The longest ending substring common to all strings

D. The longest substring common to any two strings

Solution

1. Step 1: Understand the problem definition

   The Longest Common Prefix means the longest substring at the start of all strings that is the same.

2. Step 2: Compare options with the definition

   Only The longest starting substring common to all strings matches the definition exactly: longest starting substring common to all strings.

3. Final Answer:

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

4. Quick Check:

   Longest Common Prefix = longest starting substring [OK]

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

Common Mistakes:

• Confusing prefix with substring anywhere
• Thinking about suffix instead of prefix
• Counting characters instead of substring

2. Which of the following C++ code snippets correctly initializes a vector of strings named strs with values "flower", "flow", "flight"?

easy

A. std::vector strs = {"flower", "flow", "flight"};

B. std::vector strs = {"flower", "flow", "flight"};

C. vector<string> strs = ("flower", "flow", "flight");

D. std::vector strs = ("flower", "flow", "flight");

Solution

1. Step 1: Check correct vector initialization syntax

   In C++, to initialize a vector of strings with values, use curly braces {} and specify type with std::vector.

2. Step 2: Validate each option

   std::vector strs = {"flower", "flow", "flight"}; uses correct syntax with std::vector and braces {}. Versions missing std:: namespace, using parentheses instead of braces, or lacking template type <std::string> are invalid.

3. Final Answer:

   std::vector strs = {"flower", "flow", "flight"}; -> Option A

4. Quick Check:

   Correct vector initialization uses braces and type [OK]

Hint: Use curly braces {} and full type for vector initialization [OK]

Common Mistakes:

• Using parentheses instead of braces
• Omitting std:: namespace
• Missing template argument

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

std::vector strs = {"interview", "internet", "internal"};
std::string prefix = strs[0];
for (int i = 1; i < strs.size(); i++) {
    int j = 0;
    while (j < prefix.size() && j < strs[i].size() && prefix[j] == strs[i][j]) {
        j++;
    }
    prefix = prefix.substr(0, j);
}
std::cout << prefix << std::endl;

medium

A. int

B. inte

C. intern

D. inter

Solution

1. Step 1: Initialize prefix and compare with each string

   Start prefix = "interview". Compare with "internet": common prefix is "inter". Update prefix = "inter".

2. Step 2: Compare updated prefix "inter" with "internal"

   Common prefix is "inter" again. So prefix remains "inter".

3. Final Answer:

   inter -> Option D

4. Quick Check:

   Common prefix of all = "inter" [OK]

Hint: Compare prefix with each string, shorten prefix on mismatch [OK]

Common Mistakes:

• Stopping after first comparison
• Not updating prefix correctly
• Confusing substring indices

4. Identify the bug in the following C++ code that tries to find the longest common prefix:

std::string longestCommonPrefix(std::vector& strs) {
    std::string prefix = strs[0];
    for (int i = 1; i < strs.size(); i++) {
        int j = 0;
        while (j < prefix.size() && j < strs[i].size() && prefix[j] == strs[i][j]) {
            j++;
        }
        prefix = prefix.substr(0, j);
        if (prefix.empty()) return "";
    }
    return prefix;
}

// Usage:
// std::vector words = {"dog", "racecar", "car"};
// std::cout << longestCommonPrefix(words) << std::endl;

medium

A. The function does not handle empty input vector

B. The function returns incorrect prefix for input {"dog", "racecar", "car"}

C. The while loop condition is incorrect and causes infinite loop

D. The function should return the longest common suffix instead

Solution

1. Step 1: Check empty input handling

   The code directly accesses strs[0] without checking if strs is empty, causing a runtime error (out-of-range access) for empty vectors.

2. Step 2: Verify other statements

   The loop logic correctly shortens the prefix, no infinite loop occurs, it returns prefix not suffix, and for {"dog", "racecar", "car"} it correctly returns "".

3. Final Answer:

   The function does not handle empty input vector -> Option A

4. Quick Check:

   Missing if(strs.empty()) return ""; [OK]

Hint: Test edge cases like empty lists first [OK]

Common Mistakes:

• Overlooking empty vector handling
• Misjudging the output for no common prefix cases
• Suspecting infinite loop in the while condition

5. Given a list of strings {"apple", "ape", "april", "apt"}, write a C++ function to find the longest common prefix using a vertical scanning approach. What will be the output of your function?

hard

A. "app"

B. "a"

C. "ap"

D. ""

Solution

1. Step 1: Understand vertical scanning approach

   Check characters column by column across all strings until mismatch or end of any string.

2. Step 2: Apply vertical scanning on given strings

   Index 0: all have 'a'. Index 1: all have 'p'. Index 2: "apple" has 'p', "ape" has 'e' (mismatch). Stop scanning.

3. Final Answer:

   "ap" -> Option C

4. Quick Check:

   Vertical scan stops at first mismatch, prefix = "ap" [OK]

Hint: Scan characters column-wise until mismatch found [OK]

Common Mistakes:

• Assuming prefix is longest word's start
• Stopping scan too early or too late
• Confusing vertical with horizontal scanning