Longest Common Prefix in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to return the first string as the initial prefix.

DSA C

char* longestCommonPrefix(char** strs, int strsSize) {
    if (strsSize == 0) return "";
    char* prefix = [1];
    return prefix;
}

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] instead of strs[0]

Returning NULL instead of a string

Returning an empty string "" immediately

The longest common prefix starts as the first string in the array.

2fill in blank

medium

Complete the code to loop through each string in the array.

DSA C

for (int i = [1]; i < strsSize; i++) {
    // compare prefix with strs[i]
}

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

A1

B0

CstrsSize

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Starting loop at 0 causing redundant comparison

Starting loop at strsSize causing no iteration

Starting loop at -1 causing invalid index

Start from index 1 because prefix is initialized with strs[0].

3fill in blank

hard

Fix the error in the while loop condition to compare characters correctly.

DSA C

while (prefix[j] && strs[i][j] && prefix[j] == [1]) {
    j++;
}

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

Astrs[i][j]

B'\0'

Cstrs[0][j]

Dprefix[j]

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing prefix[j] with itself

Comparing with strs[0][j] which is prefix itself

Comparing with '\0' which is end of string

We compare prefix[j] with strs[i][j] to find common characters.

4fill in blank

hard

Fill both blanks to update prefix length and terminate it correctly.

DSA C

prefix[[1]] = '\0';
int j = [2];

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

Aj

B0

C1

DstrsSize

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 instead of j to terminate prefix

Not resetting j to 0 causing wrong indexing

Using strsSize as index causing out of bounds

Set prefix[j] to '\0' to shorten prefix, and reset j to 0 for next iteration.

5fill in blank

hard

Fill all three blanks to complete the longestCommonPrefix function.

DSA C

char* longestCommonPrefix(char** strs, int strsSize) {
    if (strsSize == 0) return "";
    char* prefix = [1];
    int i = [2];
    int j = 0;
    while (i < strsSize) {
        while (prefix[j] && strs[i][j] && prefix[j] == strs[i][j]) {
            j++;
        }
        prefix[[3]] = '\0';
        i++;
        j = 0;
    }
    return prefix;
}

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

Astrs[0]

B1

Cj

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Starting i at 0 causing redundant comparison

Terminating prefix at 0 or wrong index

Not initializing prefix correctly

Initialize prefix with strs[0], start i at 1, and terminate prefix at j.

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