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 same position characters of all other strings until a mismatch is found or the end of the shortest string is reached.

intermediate

Why is it efficient to use the first string as a reference when finding the LCP?

Because the prefix must be part of the first string, using it as a reference reduces unnecessary comparisons and helps stop early when a mismatch occurs.

intermediate

What is the time complexity of the simple character-by-character LCP algorithm?

The time complexity is O(S), where S is the sum of all characters in all strings because each character is compared at most once.

beginner

What should the function return if the input list of strings is empty?

It should return an empty string since there is no common prefix without any strings.

What does the Longest Common Prefix represent in a list of strings?

A The longest substring common anywhere in all strings

B The longest substring common at the end of all strings

C The shortest string in the list

D The longest substring common at the start of all strings

The Longest Common Prefix is the longest starting substring common to all strings.

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 The shortest non-empty string

If any string is empty, the common prefix must be empty.

Which approach is simplest to find the Longest Common Prefix?

A Compare characters of the first string with all others until mismatch

B Sort strings and compare first and last only

C Use hash maps to count characters

D Reverse all strings and compare

Comparing characters of the first string with others until mismatch is the straightforward method.

What is the worst-case time complexity of the character-by-character LCP method?

A O(N^2), where N is number of strings

B O(log N)

C O(S), where S is total characters in all strings

D O(1)

Each character is compared at most once, so complexity is proportional to total characters.

If the first two strings are "flower" and "flow", what is their common prefix?

A flower

B flow

C flo

D fl

The longest common prefix is "flow" because it matches the start of both strings.

Explain how to find the Longest Common Prefix in a list of strings using character comparison.

Think about checking each character position across all strings.

Describe edge cases to consider when implementing Longest Common Prefix.

Consider what happens when input is minimal or has no overlap.

Practice

1. What does the Longest Common Prefix (LCP) problem ask for when given an array of strings?

easy

A. The longest ending substring common to all strings

B. The longest substring common to any two strings

C. The longest starting substring common to all strings

D. The shortest string in the array

Solution

1. Step 1: Understand the problem definition

   The Longest Common Prefix means the longest substring starting from the beginning that is shared by all strings in the array.

2. Step 2: Compare options with definition

   Only The longest starting substring common to all strings matches this definition exactly. Options A and C talk about suffix or any substring, and D is unrelated.

3. Final Answer:

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

4. Quick Check:

   Longest Common Prefix = longest starting substring [OK]

Hint: Focus on the start of strings for common parts [OK]

Common Mistakes:

• Confusing prefix with suffix
• Thinking about any substring instead of prefix
• Choosing shortest string instead of prefix

2. Which of the following C code snippets correctly initializes a variable to store the longest common prefix length?

easy

A. int lcp_length = 0;

B. char lcp_length = '';

C. int lcp_length = NULL;

D. string lcp_length = 0;

Solution

1. Step 1: Identify correct data type for prefix length

   The length of a prefix is a number, so it should be stored in an integer variable.

2. Step 2: Check initialization syntax

   int lcp_length = 0; correctly declares an int and initializes it to 0. char lcp_length = ''; uses char with invalid empty quotes, C uses NULL which is for pointers, and D uses string which is not a C type.

3. Final Answer:

   int lcp_length = 0; -> Option A

4. Quick Check:

   Prefix length is an integer initialized to zero [OK]

Hint: Use int for lengths, not char or string [OK]

Common Mistakes:

• Using char for numeric length
• Assigning NULL to int variable
• Using non-C types like string

3. What is the output of the following C code snippet that finds the longest common prefix length?

char *strs[] = {"flower", "flow", "flight"};
int i = 0;
while (strs[0][i] && strs[1][i] && strs[2][i] && strs[0][i] == strs[1][i] && strs[1][i] == strs[2][i]) {
    i++;
}
printf("%d", i);

medium

A. 4

B. 2

C. 3

D. 0

Solution

1. Step 1: Compare characters at each index

   Index 0: 'f' == 'f' == 'f' (true), index 1: 'l' == 'l' == 'l' (true), index 2: 'o' == 'o' == 'i' (false)

2. Step 2: Count matching prefix length

   Matching characters are at indices 0 and 1 only, so length is 2.

3. Final Answer:

   2 -> Option B

4. Quick Check:

   Matching prefix length = 2 [OK]

Hint: Check characters index by index until mismatch [OK]

Common Mistakes:

• Counting beyond first mismatch
• Confusing index with length
• Ignoring all strings in comparison

4. Identify the bug in this C code snippet that tries to find the longest common prefix of an array of strings:

int longestCommonPrefix(char **strs, int strsSize) {
    int i = 0;
    while (strs[0][i]) {
        for (int j = 1; j < strsSize; j++) {
            if (strs[j][i] != strs[0][i])
                return i;
        }
        i++;
    }
    return i;
}

medium

A. Does not initialize i before use

B. Returns i too early inside the loop

C. Uses wrong loop variable in inner loop

D. Does not check if strs[j][i] is null before comparison

Solution

1. Step 1: Analyze character comparison in inner loop

   The code compares strs[j][i] with strs[0][i] but does not check if strs[j][i] is '\0' (string end).

2. Step 2: Understand consequence of missing null check

   If strs[j] is shorter than strs[0], accessing strs[j][i] beyond its length causes undefined behavior or wrong result.

3. Final Answer:

   Does not check if strs[j][i] is null before comparison -> Option D

4. Quick Check:

   Check string length before comparing characters [OK]

Hint: Always check string end before character compare [OK]

Common Mistakes:

• Ignoring string length in loops
• Returning too early without full check
• Using wrong loop indices

5. Given an array of strings ["interview", "interval", "internal", "internet"], which of the following C code snippets correctly returns the longest common prefix string?

hard

A. int i = 0;
while (1) {
char c = strs[0][i];
if (c == '\0') break;
for (int j = 1; j < strsSize; j++) {
if (strs[j][i] != c || strs[j][i] == '\0')
goto end;
}
i++;
}
end:
char *prefix = malloc(i + 1);
strncpy(prefix, strs[0], i);
prefix[i] = '\0';
return prefix;

B. int i = 0;
while (strs[0][i]) {
for (int j = 1; j < strsSize; j++) {
if (strs[j][i] != strs[0][i])
break;
}
i++;
}
char *prefix = malloc(i + 1);
strncpy(prefix, strs[0], i);
prefix[i] = '\0';
return prefix;

C. int i = 0;
while (strs[0][i]) {
for (int j = 1; j < strsSize; j++) {
if (strs[j][i] != strs[0][i])
return NULL;
}
i++;
}
return strs[0];

D. int i = 0;
while (strs[0][i]) {
for (int j = 1; j < strsSize; j++) {
if (strs[j][i] != strs[0][i])
return "";
}
i++;
}
return strs[0];

Solution

1. Step 1: Check loop and character comparison logic

   The correct code uses while(1) with break if strs[0][i] == '\0', caches c = strs[0][i], and for each j checks strs[j][i] != c || strs[j][i] == '\0' then goto end before i++. This stops exactly at the first mismatch or shorter string.

2. Step 2: Verify prefix extraction and return

   After loop, malloc(i + 1), strncpy(prefix, strs[0], i), prefix[i] = '\0', return prefix. This creates and returns the new prefix string correctly.

3. Final Answer:

   Uses while(1), goto on mismatch/null, malloc/strncpy new prefix -> Option A

4. Quick Check:

   Check all strings char by char, then copy prefix [OK]

Hint: Use memory copy after finding prefix length [OK]

Common Mistakes:

• Not handling string ends properly
• Returning original string instead of prefix
• Not allocating memory for prefix