Why Longest Common Prefix in DSA Javascript?

The Big Idea

Discover how a simple trick can save you from endless letter-by-letter guessing!

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, especially if the list is long or words are long. You might miss a letter or compare too many times, making it frustrating and error-prone.

The Solution

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

Before vs After

✗ Before

function findPrefix(words) {
  let prefix = '';
  for (let i = 0; i < words[0].length; i++) {
    let char = words[0][i];
    for (let word of words) {
      if (word[i] !== char) return prefix;
    }
    prefix += char;
  }
  return prefix;
}

✓ After

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

What It Enables

This lets you quickly find the shared start of many words, which helps in search engines, autocomplete, and organizing data efficiently.

Real Life Example

When you type in a search box, the system suggests words that start with what you typed. Longest Common Prefix helps find those suggestions fast by identifying the common start among many options.

Key Takeaways

Manually checking letters is slow and error-prone.

Longest Common Prefix finds shared starts efficiently.

Useful for autocomplete, search, and data grouping.

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