Longest Common Prefix in DSA Java

Mental Model

Find the longest starting part that all words share exactly.

Analogy: Imagine several friends saying their names out loud. The longest common prefix is like the longest part at the start of their names that sounds the same for all of them.

words: [flower, flow, flight]
common prefix: [fl] -> rest differs

Dry Run Walkthrough

Input: words: [flower, flow, flight]

Goal: Find the longest prefix common to all words

Step 1: Start with prefix as the first word 'flower'

prefix = 'flower'

Why: We assume the whole first word is the prefix initially

Step 2: Compare prefix 'flower' with second word 'flow', shorten prefix to match

prefix = 'flow'

Why: The prefix must be common to both words, so shorten to 'flow'

Step 3: Compare prefix 'flow' with third word 'flight', shorten prefix to match

prefix = 'fl'

Why: Only 'fl' matches at the start of both 'flow' and 'flight'

Result:

prefix = 'fl'

Annotated Code

DSA Java

public class LongestCommonPrefix {
    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[] words = {"flower", "flow", "flight"};
        System.out.println(longestCommonPrefix(words));
    }
}

String prefix = strs[0];

Initialize prefix as the first word

for (int i = 1; i < strs.length; i++) {

Check each word against current prefix

while (!strs[i].startsWith(prefix)) {

Shorten prefix until it matches the start of current word

prefix = prefix.substring(0, prefix.length() - 1);

Remove last character from prefix to try shorter match

if (prefix.isEmpty()) return "";

If prefix is empty, no common prefix exists

Output

fl

Complexity Analysis

Time: O(S) where S is the sum of all characters in all words because we compare characters until mismatch

Space: O(1) because we only store the prefix string and no extra data structures

vs Alternative: Compared to checking all pairs or sorting, this approach is simple and efficient for small to medium inputs

Edge Cases

Empty array

Returns empty string immediately

DSA Java

if (strs == null || strs.length == 0) return "";

Array with one word

Returns that word as prefix

DSA Java

String prefix = strs[0];

No common prefix

Returns empty string after shortening prefix to empty

DSA Java

if (prefix.isEmpty()) return "";

When to Use This Pattern

When asked to find the longest starting part common to many strings, use the Longest Common Prefix pattern by iteratively shortening a candidate prefix.

Common Mistakes

Mistake: Not checking if prefix becomes empty and continuing to substring
Fix: Add a check to return empty string if prefix is empty

Mistake: Comparing characters one by one manually instead of using startsWith
Fix: Use built-in startsWith for cleaner and simpler code

Summary

Finds the longest starting substring common to all strings in the list.

Use when you need to find shared beginnings among multiple words or strings.

Start with the first word as prefix and shorten it until it fits all words.

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