Why Longest Common Prefix in DSA Rust?

The Big Idea

Discover how a simple trick can save you from endless letter checking!

The Scenario

Imagine you have a list of words from your friends, and you want to find the common beginning part they all share. Doing this by checking each letter one by one for every word can be tiring and confusing.

The Problem

Manually comparing each letter of every word is slow and easy to mess up. If the list is long or words are long, it takes a lot of time and you might miss differences or make mistakes.

The Solution

The Longest Common Prefix method quickly finds the shared starting letters among all words without checking every letter repeatedly. It saves time and avoids errors by smartly comparing only what is needed.

Before vs After

✗ Before

let words = vec!["flower", "flow", "flight"];
let mut prefix = String::new();
for i in 0..100 {
    let mut current_char = None;
    for word in &words {
        if i >= word.len() { break; }
        let c = word.chars().nth(i).unwrap();
        if current_char.is_none() { current_char = Some(c); }
        else if current_char.unwrap() != c { break; }
    }
    if current_char.is_some() { prefix.push(current_char.unwrap()); }
    else { break; }
}
println!("{}", prefix);

✓ After

fn longest_common_prefix(strs: &[&str]) -> String {
    if strs.is_empty() { return String::new(); }
    let mut prefix = strs[0].to_string();
    for s in strs.iter().skip(1) {
        while !s.starts_with(&prefix) {
            prefix.pop();
            if prefix.is_empty() { break; }
        }
    }
    prefix
}

let words = vec!["flower", "flow", "flight"];
println!("{}", longest_common_prefix(&words));

What It Enables

This lets you quickly find the shared start of many words, helping in tasks like autocomplete, search optimization, and DNA sequence analysis.

Real Life Example

When you type in a search box, the system suggests words that start with what you typed. Finding the longest common prefix helps the system know what letters to show next.

Key Takeaways

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

Longest Common Prefix finds shared starts efficiently.

Useful for autocomplete, search, and pattern matching.

Practice

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

easy

A. The shortest string in the list

B. The longest substring found anywhere in all strings

C. The longest starting substring common to all strings

D. The last character of the longest string

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 strings = A [OK]

Hint: Focus on start of strings, not anywhere inside [OK]

Common Mistakes:

• Confusing prefix with substring anywhere
• Choosing shortest string instead of common start
• Thinking prefix is the last part of strings

2. Which Rust code snippet correctly initializes a vector of strings for finding the longest common prefix?

easy

A. let words = vec!["flower", "flow", "flight"];

B. let words = ["flower", "flow", "flight"];

C. let words = vec!("flower", "flow", "flight");

D. let words = vec<str>["flower", "flow", "flight"];

Solution

1. Step 1: Identify correct vector initialization syntax

   In Rust, vec![...] creates a vector with elements inside square brackets.

2. Step 2: Check syntaxes

   vec![...] is correct. [...] without vec! creates an array, not a vector. vec!() uses incorrect parentheses. vec<str>[...] is invalid syntax.

3. Final Answer:

   let words = vec!["flower", "flow", "flight"]; -> Option A

4. Quick Check:

   vec![...] is correct vector syntax [OK]

Hint: Use vec![...] to create vectors in Rust [OK]

Common Mistakes:

• Using parentheses instead of square brackets in vec!
• Confusing arrays with vectors
• Incorrect generic syntax for vec

3. What is the output of this Rust code that finds the longest common prefix?

fn longest_common_prefix(strs: Vec<&str>) -> String {
    if strs.is_empty() { return String::new(); }
    let mut prefix = strs[0].to_string();
    for s in &strs[1..] {
        while !s.starts_with(&prefix) {
            prefix.pop();
            if prefix.is_empty() { return String::new(); }
        }
    }
    prefix
}

fn main() {
    let words = vec!["interview", "internet", "internal"];
    println!("{}", longest_common_prefix(words));
}

medium

A. "int"

B. "inter"

C. "in"

D. "internal"

Solution

1. Step 1: Initialize prefix with first string

   Prefix starts as "interview".

2. Step 2: Compare prefix with second string "internet"

   "internet".starts_with("interview") is false, so pop last char repeatedly: pop 'w' -> "intervie" (false), pop 'e' -> "intervi" (false), pop 'i' -> "interv" (false), pop 'v' -> "inter" (true, since "internet".starts_with("inter")).

3. Step 3: Compare prefix "inter" with third string "internal"

   "internal".starts_with("inter") is true, so prefix remains "inter".

4. Final Answer:

   "inter" -> Option B

5. Quick Check:

   Common start is "inter" = B [OK]

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

Common Mistakes:

• Not shrinking prefix correctly
• Assuming full first string is prefix
• Confusing substring anywhere with prefix

4. Identify the bug in this Rust function for longest common prefix and select the fix:

fn longest_common_prefix(strs: Vec<&str>) -> String {
    if strs.is_empty() { return String::new(); }
    let mut prefix = strs[0].to_string();
    for s in &strs[1..] {
        while !s.starts_with(prefix) {
            prefix.pop();
            if prefix.is_empty() { return String::new(); }
        }
    }
    prefix
}

medium

A. Change s.starts_with(prefix) to s.starts_with(&prefix)

B. Replace prefix.pop() with prefix.remove(0)

C. Initialize prefix as empty string instead of strs[0]

D. Add a break statement inside the while loop

Solution

1. Step 1: Identify type mismatch in starts_with

   starts_with expects a &str reference, but prefix is String. Passing prefix directly causes type error.

2. Step 2: Fix by passing reference to prefix

   Use &prefix to pass a &str reference, matching function signature.

3. Final Answer:

   Change s.starts_with(prefix) to s.starts_with(&prefix) -> Option A

4. Quick Check:

   Use & prefix for &str parameter [OK]

Hint: Pass &String as &str in starts_with [OK]

Common Mistakes:

• Passing String instead of &str to starts_with
• Removing first char instead of last char
• Not checking empty prefix before popping

5. Given a list of strings, write a Rust function that returns the longest common prefix but ignores case differences (case-insensitive). Which approach is best?

hard

A. Reverse strings and find prefix from the end

B. Compare characters using ASCII values directly

C. Use starts_with without modification

D. Convert all strings to lowercase before finding prefix

Solution

1. Step 1: Understand case-insensitive comparison

   To ignore case, convert all strings to the same case (lowercase or uppercase) before comparison.

2. Step 2: Apply lowercase conversion before prefix logic

   Transform each string to lowercase, then find longest common prefix normally.

3. Final Answer:

   Convert all strings to lowercase before finding prefix -> Option D

4. Quick Check:

   Normalize case first for case-insensitive prefix [OK]

Hint: Normalize case before prefix check [OK]

Common Mistakes:

• Comparing characters without case normalization
• Using ASCII values without case mapping
• Reversing strings which doesn't solve case issue