Why Longest Common Prefix in DSA C#?

The Big Idea

Discover how a simple trick can save you hours of tedious word comparisons!

The Scenario

Imagine you have a list of words like "flower", "flow", "flight" and you want to find the part they all start with. Doing this by checking each letter one by one for every word can be tiring and confusing.

The Problem

Manually comparing each letter across all words is slow and easy to mess up. You might miss differences or spend too much time going back and forth, especially if the list is long or words are long.

The Solution

The Longest Common Prefix method quickly finds the shared beginning part of all words by smartly comparing characters only as needed. It saves time and avoids mistakes by stopping as soon as a difference is found.

Before vs After

✗ Before

string prefix = "";
for (int i = 0; i < words[0].Length; i++) {
    char c = words[0][i];
    foreach (string word in words) {
        if (i == word.Length || word[i] != c) return prefix;
    }
    prefix += c;
}

✓ After

string LongestCommonPrefix(string[] words) {
    if (words.Length == 0) return "";
    string prefix = words[0];
    foreach (string word in words) {
        while (!word.StartsWith(prefix)) {
            prefix = prefix.Substring(0, prefix.Length - 1);
            if (prefix == "") return "";
        }
    }
    return prefix;
}

What It Enables

This lets you quickly find the shared start of many words, which helps in search, autocomplete, 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 that shared start fast.

Key Takeaways

Manually checking each letter is slow and error-prone.

Longest Common Prefix finds the shared start efficiently.

It helps in search, autocomplete, and organizing words.

Practice

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

easy

A. The longest starting substring common to all strings

B. The longest substring found anywhere in all strings

C. The shortest string in the array

D. The number of strings in the array

Solution

1. Step 1: Understand the problem definition

   The Longest Common Prefix problem asks for the longest substring that appears at the start of every string in the array.

2. Step 2: Compare options with the definition

   The longest starting substring common to all strings matches the definition exactly. Options A, B, and C do not describe the prefix or the problem correctly.

3. Final Answer:

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

4. Quick Check:

   Longest Common Prefix = longest starting substring [OK]

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

Common Mistakes:

• Confusing prefix with substring anywhere
• Thinking about shortest string instead
• Counting strings instead of prefix

2. Which of the following C# method signatures is correct for a function that finds the longest common prefix of a string array?

easy

A. public void LongestCommonPrefix(string[] strs)

B. public int LongestCommonPrefix(string[] strs)

C. public string LongestCommonPrefix(string[] strs)

D. public string LongestCommonPrefix(List<string> strs)

Solution

1. Step 1: Identify the expected return type

   The longest common prefix is a string, so the method should return a string.

2. Step 2: Check parameter type and return type

   public string LongestCommonPrefix(string[] strs) returns string and takes string array, which is correct. public int LongestCommonPrefix(string[] strs) returns int (wrong type). public void LongestCommonPrefix(string[] strs) returns void (no return). public string LongestCommonPrefix(List<string> strs) uses List<string> instead of array, which is valid but not matching the question's array input.

3. Final Answer:

   public string LongestCommonPrefix(string[] strs) -> Option C

4. Quick Check:

   Return type string with string[] parameter [OK]

Hint: Return string, input string array for LCP method [OK]

Common Mistakes:

• Using wrong return type like int or void
• Using List<string> instead of string[] when array expected
• Confusing parameter types

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

string[] words = {"flower", "flow", "flight"};
string prefix = LongestCommonPrefix(words);
Console.WriteLine(prefix);

string LongestCommonPrefix(string[] strs) {
    if (strs == null || strs.Length == 0) return "";
    string prefix = strs[0];
    for (int i = 1; i < strs.Length; i++) {
        while (!strs[i].StartsWith(prefix)) {
            prefix = prefix.Substring(0, prefix.Length - 1);
            if (prefix == "") return "";
        }
    }
    return prefix;
}

medium

A. "fl"

B. "flow"

C. "f"

D. "flight"

Solution

1. Step 1: Initialize prefix and iterate

   Prefix starts as "flower". Compare with "flow": "flow" does not start with "flower", so remove last char -> "flowe", then "flow", which matches.

2. Step 2: Compare prefix with "flight"

   "flight" does not start with "flow", remove last char: "flo", no; "fl", yes. So prefix becomes "fl".

3. Final Answer:

   "fl" -> Option A

4. Quick Check:

   Common prefix of all = "fl" [OK]

Hint: Shrink prefix until all strings start with it [OK]

Common Mistakes:

• Assuming prefix is first string always
• Not shrinking prefix correctly
• Returning empty string too early

4. The following code attempts to find the longest common prefix but has a bug. What is the bug?

string LongestCommonPrefix(string[] strs) {
    if (strs == null || strs.Length == 0) return "";
    string prefix = strs[0];
    for (int i = 1; i < strs.Length; i++) {
        while (!strs[i].Contains(prefix)) {
            prefix = prefix.Substring(0, prefix.Length - 1);
            if (prefix == "") return "";
        }
    }
    return prefix;
}

medium

A. Not checking for null strings inside array

B. Using Contains instead of StartsWith causes wrong prefix check

C. Prefix is not updated inside the loop

D. Loop index starts from 0 instead of 1

Solution

1. Step 1: Analyze the prefix check method

   The code uses strs[i].Contains(prefix) to check prefix, but prefix must be at the start, so StartsWith is correct.

2. Step 2: Understand impact of Contains

   Contains allows prefix anywhere in the string, so it may accept wrong prefixes that are not at the start.

3. Final Answer:

   Using Contains instead of StartsWith causes wrong prefix check -> Option B

4. Quick Check:

   Prefix must be at start, not anywhere [OK]

Hint: Use StartsWith, not Contains, to check prefix [OK]

Common Mistakes:

• Using Contains instead of StartsWith
• Ignoring null or empty strings in array
• Incorrect loop start index

5. Given the array ["interview", "interrupt", "integrate", "integral"], what is the longest common prefix? Also, explain why the prefix ends where it does.

hard

A. "integ"

B. "inte"

C. "inter"

D. "int"

Solution

1. Step 1: Compare strings character by character

   All strings start with "int". "interview" and "interrupt" continue with "er", while "integrate" and "integral" continue with "eg".

2. Step 2: Identify where prefix breaks

   "interview" and "interrupt" have "inter", but "integrate" and "integral" have "integ", so common prefix stops at "int".

3. Final Answer:

   "int" -> Option D

4. Quick Check:

   Prefix stops where any string differs [OK]

Hint: Prefix ends at first differing character among strings [OK]

Common Mistakes:

• Assuming longer prefix without checking all strings
• Ignoring difference in later strings
• Confusing similar prefixes with actual common prefix