Longest Common Prefix in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to return an empty string if the input array is empty.

DSA Typescript

if (strs.length === 0) return [1];

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

A""

Bundefined

Cnull

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Returning null or undefined instead of an empty string.

Returning 0 which is not a string.

If the array is empty, the longest common prefix is an empty string.

2fill in blank

medium

Complete the code to initialize the prefix with the first string in the array.

DSA Typescript

let prefix = [1];

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

A""

Bstrs[1]

Cstrs[0]

Dstrs[strs.length - 1]

Attempts:

3 left

💡 Hint

Common Mistakes

Starting with an empty string which will always result in empty prefix.

Using the last string or second string which is not standard.

We start with the first string as the initial prefix to compare with others.

3fill in blank

hard

Fix the error in the while loop condition to check if the current string starts with the prefix.

DSA Typescript

while (!strs[i].[1](prefix)) {

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

AindexOf

BendsWith

Cincludes

DstartsWith

Attempts:

3 left

💡 Hint

Common Mistakes

Using endsWith which checks the suffix, not prefix.

Using includes or indexOf which check anywhere in the string.

We need to check if the string starts with the prefix to find the common prefix.

4fill in blank

hard

Fill both blanks to shorten the prefix by one character from the end inside the while loop.

DSA Typescript

prefix = prefix[1](0, prefix[2] - 1);

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

Aslice

Blength

Csubstr

Dsubstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using substr which has different parameters.

Using substring but mixing parameters incorrectly.

We use slice(0, length - 1) to remove the last character from prefix.

5fill in blank

hard

Fill all three blanks to complete the for loop that iterates over the strings starting from the second one.

DSA Typescript

for (let i = [1]; i < strs[2]; i[3]) {

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

A1

B.length

C++

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Starting loop at 0 which is the first string already used as prefix.

Using i-- or other incorrect increments.

The loop starts at 1, runs while i < strs.length, and increments i by 1 each time.

Practice

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

easy

A. The longest starting substring common to all strings

B. The longest ending substring common to all strings

C. The longest substring found anywhere in all strings

D. The shortest string in the list

Solution

1. Step 1: Understand the prefix concept

   A prefix is a substring at the start of a string.

2. Step 2: Define longest common prefix

   The LCP is the longest substring starting from the beginning that all strings share.

3. Final Answer:

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

4. Quick Check:

   Prefix means start of string = B [OK]

Hint: Focus on the start of strings to find common parts [OK]

Common Mistakes:

• Confusing prefix with suffix
• Looking for substring anywhere, not just start
• Choosing shortest string as prefix

2. Which of the following TypeScript function signatures correctly represents a function to find the longest common prefix of an array of strings?

easy

A. function longestCommonPrefix(strs: string[]): string

B. function longestCommonPrefix(strs: number[]): string

C. function longestCommonPrefix(strs: string): string[]

D. function longestCommonPrefix(strs: string[]): number

Solution

1. Step 1: Identify input type

   The input should be an array of strings, so type is string[].

2. Step 2: Identify output type

   The output is the longest common prefix, a string.

3. Final Answer:

   function longestCommonPrefix(strs: string[]): string -> Option A

4. Quick Check:

   Input array of strings, output string = D [OK]

Hint: Input is string array, output is string prefix [OK]

Common Mistakes:

• Using number[] instead of string[]
• Returning array instead of string
• Passing single string instead of array

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

function longestCommonPrefix(strs: string[]): string {
  if (strs.length === 0) return "";
  let prefix = strs[0];
  for (let i = 1; i < strs.length; i++) {
    while (strs[i].indexOf(prefix) !== 0) {
      prefix = prefix.substring(0, prefix.length - 1);
      if (prefix === "") return "";
    }
  }
  return prefix;
}

console.log(longestCommonPrefix(["flower", "flow", "flight"]));

medium

A. "flow"

B. "fl"

C. "f"

D. "flight"

Solution

1. Step 1: Initialize prefix and loop through strings

   Start with prefix = "flower". Check "flow": prefix "flower" not prefix of "flow", shorten to "flowe", then "flow", which is prefix of "flow".

2. Step 2: Check next string "flight"

   "flow" not prefix of "flight", shorten to "flo", then "fl", which is prefix of "flight".

3. Final Answer:

   "fl" -> Option B

4. Quick Check:

   Common start is "fl" = A [OK]

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

Common Mistakes:

• Returning first string without checking others
• Not shortening prefix correctly
• Confusing substring indices

4. Identify the bug in this TypeScript function for longest common prefix:

function longestCommonPrefix(strs: string[]): string {
  if (strs.length === 0) return "";
  let prefix = strs[0];
  for (let i = 1; i <= strs.length; i++) {
    while (strs[i].indexOf(prefix) !== 0) {
      prefix = prefix.substring(0, prefix.length - 1);
      if (prefix === "") return "";
    }
  }
  return prefix;
}

medium

A. Use prefix.includes() instead of indexOf()

B. Prefix should be initialized as empty string

C. Loop condition should be i < strs.length, not i <= strs.length

D. Return type should be string[] instead of string

Solution

1. Step 1: Check loop boundary

   The loop uses i <= strs.length, which causes out-of-bounds access at i = strs.length.

2. Step 2: Correct loop condition

   Change to i < strs.length to avoid accessing undefined element.

3. Final Answer:

   Loop condition should be i < strs.length, not i <= strs.length -> Option C

4. Quick Check:

   Array index out of bounds = C [OK]

Hint: Use < not <= for zero-based array loops [OK]

Common Mistakes:

• Off-by-one error in loop condition
• Initializing prefix incorrectly
• Misusing string methods

5. Given the array ["interview", "interval", "internal", "internet"], what is the longest common prefix? Also, explain why the prefix stops where it does.

hard

A. "inte" because all strings start with these four letters

B. "in" because only first two letters are common

C. "int" because the sixth letter differs in some strings

D. "inter" because all strings start with these five letters

Solution

1. Step 1: Compare characters one by one

   Check each character index across all strings: 'i', 'n', 't', 'e', 'r' are common in all.

2. Step 2: Identify where mismatch occurs

   At index 5, 'v' in "interview" and "interval", but 'n' in "internal" and "internet", so prefix stops at "inter".

3. Final Answer:

   "inter" because all strings start with these five letters -> Option D

4. Quick Check:

   Common start "inter" = B [OK]

Hint: Check each character position until mismatch found [OK]

Common Mistakes:

• Assuming prefix is longer than actual common part
• Ignoring character positions carefully
• Confusing similar but different letters