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.

beginner

How can you find the Longest Common Prefix by comparing characters?

Compare characters of the first string with the corresponding characters of all other strings until a mismatch is found or the end of any string is reached.

intermediate

What is the time complexity of the straightforward approach to find the Longest Common Prefix?

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

beginner

Why is it important to consider the shortest string length when finding the Longest Common Prefix?

Because the prefix cannot be longer than the shortest string in the list.

beginner

What happens if the list of strings is empty when finding the Longest Common Prefix?

The Longest Common Prefix is an empty string since there are no strings to compare.

What is the Longest Common Prefix of ["flower", "flow", "flight"]?

A "flow"

B "fl"

C "f"

D "flo"

All strings start with "fl", but "flo" is not common to "flight".

If one string in the list is empty, what is the Longest Common Prefix?

A Empty string

B The first string

C The longest string

D None of the above

An empty string means no common prefix can exist beyond empty.

Which approach is NOT used to find the Longest Common Prefix?

A Horizontal scanning

B Vertical scanning

C Binary search on prefix length

D Sorting strings by length only

Sorting by length alone does not help find the prefix.

What is the first step in the vertical scanning method for LCP?

A Compare characters column-wise across all strings

B Sort strings alphabetically

C Find the shortest string

D Concatenate all strings

Vertical scanning compares characters at the same index in all strings.

What is the output of LCP for ["dog", "racecar", "car"]?

A "d"

B "c"

C "" (empty string)

D "r"

No common prefix exists among these strings.

Explain how to find the Longest Common Prefix using the horizontal scanning method.

Think of comparing pairs of strings one by one.

Describe edge cases to consider when implementing Longest Common Prefix.

Think about unusual or minimal inputs.

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