Longest Common Prefix in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA Javascript

function longestCommonPrefix(strs) {
  if (strs.length === 0) return [1];
  return "";
}

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

A""

B0

Cundefined

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Returning null or undefined instead of an empty string.

Returning 0 which is not a string.

If the input array is empty, the longest common prefix is an empty string, so we return "".

2fill in blank

medium

Complete the code to set the prefix to the first string in the array.

DSA Javascript

function longestCommonPrefix(strs) {
  if (strs.length === 0) return "";
  let prefix = [1];
  return prefix;
}

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

Astrs[0]

Bstrs[1]

Cstrs[strs.length - 1]

D""

Attempts:

3 left

💡 Hint

Common Mistakes

Using strs[1] or last string which may not exist or is not the best start.

Starting with an empty string which will always return empty.

We start with the first string as the prefix and then compare it with others to shorten it if needed.

3fill in blank

hard

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

DSA Javascript

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

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

AstartsWith

BindexOf

Cincludes

Dsearch

Attempts:

3 left

💡 Hint

Common Mistakes

Using startsWith which returns true/false, not a number.

Using includes which checks anywhere in the string.

indexOf(prefix) === 0 means the string starts with the prefix. This is the correct check.

4fill in blank

hard

Fill both blanks to correctly shorten the prefix inside the loop.

DSA Javascript

function longestCommonPrefix(strs) {
  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[1](0, prefix[2]);
      if (prefix === "") return "";
    }
  }
  return prefix;
}

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

A.slice

B.substring

C.substr

D.length - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using split which returns an array, not a string.

Using substring or substr incorrectly without adjusting length.

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

5fill in blank

hard

Fill all three blanks to complete the function that finds the longest common prefix.

DSA Javascript

function longestCommonPrefix(strs) {
  if (strs.length === 0) return [1];
  let prefix = strs[0];
  for (let i = 1; i < strs.length; i++) {
    while (strs[i].indexOf(prefix) !== 0) {
      prefix = prefix.slice(0, prefix.length [2] 1);
      if (prefix === [3]) return "";
    }
  }
  return prefix;
}

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

A""

B-

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Returning null instead of empty string.

Using '+' instead of '-' causing wrong slice length.

Return empty string if no input, shorten prefix by subtracting 1, and check if prefix is empty string.

Practice

1. What does the Longest Common Prefix (LCP) problem ask you to find given an array of strings?

easy

A. The longest substring common to any two strings

B. The longest starting substring common to all strings

C. The longest ending substring common to all strings

D. The shortest substring unique to each string

Solution

1. Step 1: Understand the problem definition

   The Longest Common Prefix means the longest substring at the start of all strings that is the same.

2. Step 2: Compare options with definition

   Only The longest starting substring common to all strings matches this definition exactly.

3. Final Answer:

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

4. Quick Check:

   Longest Common Prefix = longest starting substring [OK]

Hint: Focus on the start of strings for common prefix [OK]

Common Mistakes:

• Confusing prefix with suffix
• Looking for substring anywhere, not just start
• Thinking about unique substrings

2. Which of the following is the correct JavaScript syntax to start a function named longestCommonPrefix that takes an array strs as input?

easy

A. function: longestCommonPrefix(strs) { }

B. func longestCommonPrefix(strs) { }

C. function longestCommonPrefix(strs) { }

D. def longestCommonPrefix(strs):

Solution

1. Step 1: Recall JavaScript function syntax

   JavaScript functions start with the keyword 'function' followed by name and parentheses.

2. Step 2: Check each option

   function longestCommonPrefix(strs) { } matches JavaScript syntax. Options B and C are from other languages. function: longestCommonPrefix(strs) { } is invalid syntax.

3. Final Answer:

   function longestCommonPrefix(strs) { } -> Option C

4. Quick Check:

   JS function syntax = function name(params) { } [OK]

Hint: JS functions start with 'function' keyword [OK]

Common Mistakes:

• Using Python or other language syntax
• Adding colon after function name
• Using 'func' instead of 'function'

3. What will be the output of the following JavaScript code?

function longestCommonPrefix(strs) {
  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. "fl"

B. "flow"

C. "f"

D. "flower"

Solution

1. Step 1: Initialize prefix and loop through strings

   Prefix starts as "flower". Check "flow": "flower" is not prefix, shorten to "flowe", then "flow", which matches start of "flow".

2. Step 2: Check next string "flight"

   "flow" is not prefix of "flight", shorten to "flo", no match; then "fl", which matches start of "flight".

3. Final Answer:

   "fl" -> Option A

4. Quick Check:

   Common prefix of flower, flow, flight = "fl" [OK]

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

Common Mistakes:

• Returning first string as prefix
• Not shrinking prefix correctly
• Confusing substring with suffix

4. Find the bug in this JavaScript function for Longest Common Prefix:

function longestCommonPrefix(strs) {
  if (!strs.length) 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. The for loop condition should be i < strs.length, not i <= strs.length

B. The prefix should be initialized as an empty string

C. The while loop condition should use includes() instead of indexOf()

D. The function should return prefix inside the for loop

Solution

1. Step 1: Check for loop bounds

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

2. Step 2: Correct loop condition

   Changing to i < strs.length prevents accessing undefined strs[i].

3. Final Answer:

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

4. Quick Check:

   Loop index must be less than array length [OK]

Hint: Use < not <= for array indices [OK]

Common Mistakes:

• Off-by-one error in loop
• Wrong initialization of prefix
• Misusing string methods

5. Given an array of strings, how would you modify the Longest Common Prefix algorithm to handle the case where some strings are empty or the array is empty, ensuring no errors and correct output?

hard

A. Throw an error if any string is empty

B. Ignore empty strings and find prefix among non-empty strings only

C. Replace empty strings with the first string in the array before processing

D. Return an empty string immediately if the array is empty or any string is empty

Solution

1. Step 1: Check for empty array or empty strings

   If the array is empty or any string is empty, the longest common prefix must be empty string.

2. Step 2: Return empty string early

   Returning "" immediately avoids errors and matches expected output.

3. Final Answer:

   Return an empty string immediately if the array is empty or any string is empty -> Option D

4. Quick Check:

   Empty input means empty prefix [OK]

Hint: Check for empty inputs first [OK]

Common Mistakes:

• Not handling empty strings causing errors
• Ignoring empty strings and causing wrong prefix
• Throwing errors instead of returning empty string