Why Longest Common Prefix in DSA C?

The Big Idea

Discover how a simple trick can save you from endless letter checking!

The Scenario

Imagine you have a list of words, like names of fruits: apple, apricot, and ape. You want to find the common starting letters they all share. Doing this by checking each letter one by one for every word is like looking through a messy drawer trying to find matching socks by hand.

The Problem

Manually comparing each letter for every word is slow and tiring. It's easy to make mistakes, especially if the list is long or words are long. You might miss a letter or compare too many times, wasting time and effort.

The Solution

The Longest Common Prefix method quickly finds the shared starting letters among all words. It smartly compares letters only as needed, stopping as soon as a difference is found. This saves time and avoids errors, like having a neat organizer for your socks.

Before vs After

✗ Before

int i = 0;
while (all_words_have_same_char_at(i)) {
    i++;
}
return prefix_up_to(i);

✓ After

char* longestCommonPrefix(char** words, int count) {
    if (count == 0) return "";
    for (int i = 0; words[0][i]; i++) {
        for (int j = 1; j < count; j++) {
            if (!words[j][i] || words[j][i] != words[0][i]) {
                ((char*)words[0])[i] = '\0';
                return words[0];
            }
        }
    }
    return words[0];
}

What It Enables

This lets you quickly find the shared start of many words, helping in tasks like auto-complete, search optimization, and grouping similar items.

Real Life Example

When you type in a search box, the system suggests words that start the same way you typed. Longest Common Prefix helps find those suggestions fast by identifying the shared beginning letters.

Key Takeaways

Manual letter-by-letter checks are slow and error-prone.

Longest Common Prefix finds shared starts efficiently.

It powers features like search suggestions and grouping.

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