Why Longest Common Prefix in DSA Typescript?

The Big Idea

Discover how a simple trick saves you from endless letter comparisons!

The Scenario

Imagine you have a list of words like 'flower', 'flow', and 'flight'. You want to find the common beginning part of these words by checking each letter one by one manually.

The Problem

Doing this by hand or with simple loops is slow and error-prone because you have to compare each character of every word repeatedly. It's easy to miss differences or waste time checking unnecessary letters.

The Solution

The Longest Common Prefix method quickly finds the shared starting letters among all words by smartly comparing characters only as needed, saving time and avoiding mistakes.

Before vs After

✗ Before

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

✓ After

function longestCommonPrefix(words: string[]): string {
  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 is useful in search, autocomplete, and grouping similar items.

Real Life Example

When you type in a search box, the system suggests words that start the same way you typed. Longest Common Prefix helps find that shared start fast.

Key Takeaways

Manual letter-by-letter checks are slow and error-prone.

Longest Common Prefix efficiently finds shared beginnings among words.

This method speeds up tasks like autocomplete and grouping.

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