Longest Common Prefix in DSA Java - Build from Scratch

Longest Common Prefix

📖 Scenario: Imagine you work in a company that manages a list of product codes. You want to find the longest starting part (prefix) that all product codes share. This helps in grouping similar products quickly.

🎯 Goal: Build a Java program that finds the longest common prefix string among an array of product codes.

📋 What You'll Learn

Create an array of strings called products with exact values

Create a variable called prefix to store the longest common prefix

Use a loop to compare characters of the strings to find the prefix

Print the longest common prefix

💡 Why This Matters

🌍 Real World

Finding common prefixes helps in grouping similar items, like product codes or file paths, which is useful in inventory management and search optimization.

💼 Career

This concept is important for software developers working on text processing, search engines, and data grouping tasks.

1

Create the product codes array

Create a String array called products with these exact values: "flower", "flow", "flight".

DSA Java

// Create the products array with the given values
// Your code here

Hint

Use String[] products = {"flower", "flow", "flight"}; to create the array.

2

Create the prefix variable

Create a String variable called prefix and set it to the first string in products (i.e., products[0]).

DSA Java

public class Main {
    public static void main(String[] args) {
        String[] products = {"flower", "flow", "flight"};
        // Create prefix variable and set it to products[0]
        // Your code here
    }
}

Hint

Use String prefix = products[0]; to start with the first product code.

3

Find the longest common prefix

Use a for loop with variable i from 1 to products.length to compare prefix with each string in products. Inside the loop, use a while loop to shorten prefix until it matches the start of products[i] using startsWith.

DSA Java

public class Main {
    public static void main(String[] args) {
        String[] products = {"flower", "flow", "flight"};
        String prefix = products[0];
        // Use a for loop to compare prefix with each product
        // Use a while loop inside to shorten prefix if needed
        // Your code here
    }
}

Hint

Use startsWith to check if the current product starts with prefix. Shorten prefix by removing the last character until it matches.

4

Print the longest common prefix

Print the value of prefix using System.out.println(prefix);.

DSA Java

public class Main {
    public static void main(String[] args) {
        String[] products = {"flower", "flow", "flight"};
        String prefix = products[0];
        for (int i = 1; i < products.length; i++) {
            while (!products[i].startsWith(prefix)) {
                prefix = prefix.substring(0, prefix.length() - 1);
                if (prefix.isEmpty()) {
                    break;
                }
            }
        }
        // Print the prefix
        // Your code here
    }
}

Hint

Use System.out.println(prefix); to display the longest common prefix.

Practice

1. What does the Longest Common Prefix (LCP) of a list of strings represent?

easy

A. The longest substring found anywhere in all strings

B. The longest ending substring common to all strings

C. The longest starting substring common to all strings

D. The shortest substring common to any two strings

Solution

1. Step 1: Understand the prefix concept

   A prefix is a substring starting at the beginning of a string.

2. Step 2: Define longest common prefix

   The longest common prefix is the longest substring starting at the beginning that all strings share.

3. Final Answer:

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

4. Quick Check:

   Prefix means start of string = longest common start [OK]

Hint: Longest common prefix starts at string beginnings [OK]

Common Mistakes:

• Confusing prefix with substring anywhere
• Thinking it is the longest suffix
• Mixing prefix with shortest substring

2. Which of the following Java method signatures is correct for a function that finds the longest common prefix of an array of strings named strs?

easy

A. public String longestCommonPrefix(String[] strs)

B. public void longestCommonPrefix(String[] strs)

C. public int longestCommonPrefix(String[] strs)

D. public String longestCommonPrefix(List<String> strs)

Solution

1. Step 1: Identify return type for prefix

   The longest common prefix is a string, so return type must be String.

2. Step 2: Check parameter type

   The input is an array of strings, so parameter type is String[].

3. Final Answer:

   public String longestCommonPrefix(String[] strs) -> Option A

4. Quick Check:

   Return String, input String[] = public String longestCommonPrefix(String[] strs) [OK]

Hint: Return type matches prefix type; input is String array [OK]

Common Mistakes:

• Using void return type instead of String
• Returning int instead of String
• Using List<String> instead of String[]

3. What is the output of the following Java code?

public class Test {
  public static 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.isEmpty()) return "";
      }
    }
    return prefix;
  }
  public static void main(String[] args) {
    String[] input = {"flower", "flow", "flight"};
    System.out.println(longestCommonPrefix(input));
  }
}

medium

A. "flow"

B. "fl"

C. "f"

D. ""

Solution

1. Step 1: Initialize prefix and check each string

   Start with prefix = "flower". Check "flow": "flow" starts with "flower"? No, shorten prefix to "flowe", then "flow", which matches.

2. Step 2: Check next string "flight"

   "flight" starts with "flow"? No, shorten prefix to "flo", then "fl", which matches.

3. Final Answer:

   "fl" -> Option B

4. Quick Check:

   Common start of all strings = "fl" [OK]

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

Common Mistakes:

• Returning first string without checking others
• Not shortening prefix correctly
• Returning empty string too early

4. Identify the error in the following Java method for finding the longest common prefix:

public String longestCommonPrefix(String[] strs) {
  if (strs.length == 0) return null;
  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.isEmpty()) return "";
    }
  }
  return prefix;
}

medium

A. The substring method is used incorrectly

B. The method should return null instead of empty string when no prefix

C. The prefix should be initialized as an empty string

D. The loop condition should be i < strs.length, not i <= strs.length

Solution

1. Step 1: Check loop bounds

   The for loop uses i <= strs.length, which causes ArrayIndexOutOfBoundsException because valid indices are 0 to strs.length - 1.

2. Step 2: Correct loop condition

   Change to i < strs.length to avoid accessing invalid index.

3. Final Answer:

   The loop condition should be i < strs.length, not i <= strs.length -> Option D

4. Quick Check:

   Array indices go from 0 to length-1 [OK]

Hint: Loop index must be less than array length [OK]

Common Mistakes:

• Using i <= length causing runtime error
• Returning null instead of empty string
• Incorrect substring usage

5. Given the array String[] words = {"interview", "interrupt", "integrate", "integral"};, which of the following modifications to the longest common prefix algorithm will correctly find the prefix "int" even if some strings are empty or null?

hard

A. Add checks to skip null or empty strings before comparing prefixes

B. Return the first string immediately if any string is null or empty

C. Ignore the first string and start prefix from the second string

D. Use the longest string as the initial prefix instead of the first string

Solution

1. Step 1: Understand problem with null or empty strings

   Null or empty strings cause errors or incorrect prefix results if not handled.

2. Step 2: Modify algorithm to skip or handle null/empty

   Check each string before comparing; skip or handle null/empty to avoid errors and find correct prefix.

3. Final Answer:

   Add checks to skip null or empty strings before comparing prefixes -> Option A

4. Quick Check:

   Handle null/empty strings safely to find correct prefix [OK]

Hint: Always check for null or empty strings before prefix checks [OK]

Common Mistakes:

• Not handling null causing exceptions
• Returning first string without checks
• Starting prefix from wrong string