Longest Common Prefix in DSA Go - Build from Scratch

Longest Common Prefix Finder

📖 Scenario: Imagine you are building a feature for a search engine that suggests common beginnings of words typed by users. To do this, you need to find the longest common prefix among a list of words.

🎯 Goal: Build a Go program that finds the longest common prefix string among a list of words.

📋 What You'll Learn

Create a slice of strings with exact words

Create a variable to hold the longest common prefix

Write a function to find the longest common prefix among the words

Print the longest common prefix

💡 Why This Matters

🌍 Real World

Finding common prefixes is useful in search engines, autocomplete features, and DNA sequence analysis.

💼 Career

Understanding string manipulation and writing functions is essential for software development and data processing roles.

1

Create the list of words

Create a slice of strings called words with these exact values: "flower", "flow", "flight".

DSA Go

words := []string{"flower", "flow", "flight"}  // Your code here

Hint

Use a slice literal to create words with the exact strings.

2

Create a variable for the prefix

Create a variable called prefix of type string and set it to an empty string "".

DSA Go

package main

func main() {
    words := []string{"flower", "flow", "flight"}
    prefix := ""  // Your code here
}

Hint

Declare prefix as a string and assign it an empty string.

3

Write the function to find the longest common prefix

Write a function called longestCommonPrefix that takes a slice of strings called strs and returns a string. Inside main, assign prefix to the result of calling longestCommonPrefix(words). The function should find the longest common prefix among the strings in strs.

DSA Go

package main

func longestCommonPrefix(strs []string) string {
    // Your code here
}

func main() {
    words := []string{"flower", "flow", "flight"}
    prefix := ""
    prefix = longestCommonPrefix(words)  // Your code here
}

Hint

Use the first string as the initial prefix. Compare it with each string and shorten the prefix until it matches all.

4

Print the longest common prefix

Write a fmt.Println statement to print the value of prefix.

DSA Go

package main

import "fmt"

func longestCommonPrefix(strs []string) string {
    if len(strs) == 0 {
        return ""
    }
    prefix := strs[0]
    for i := 1; i < len(strs); i++ {
        j := 0
        for j < len(prefix) && j < len(strs[i]) && prefix[j] == strs[i][j] {
            j++
        }
        prefix = prefix[:j]
        if prefix == "" {
            break
        }
    }
    return prefix
}

func main() {
    words := []string{"flower", "flow", "flight"}
    prefix := ""
    prefix = longestCommonPrefix(words)
    fmt.Println(prefix)  // Your code here
}

Hint

Use fmt.Println(prefix) to display the result.

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 substring unique to each 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 A

4. Quick Check:

   Prefix means start of string = longest common start substring [OK]

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

Common Mistakes:

• Confusing prefix with substring anywhere
• Thinking about suffix instead of prefix
• Assuming shortest substring is the answer

2. Which of the following Go code snippets correctly initializes a slice of strings named strs with values "flower", "flow", "flight"?

easy

A. var strs = {"flower", "flow", "flight"}

B. strs := [3]string{"flower", "flow", "flight"}

C. strs := []string{"flower", "flow", "flight"}

D. strs := string{"flower", "flow", "flight"}

Solution

1. Step 1: Identify correct slice initialization syntax

   In Go, slices are initialized with []type{values} syntax.

2. Step 2: Check each option

   strs := []string{"flower", "flow", "flight"} uses []string with curly braces correctly. strs := [3]string{"flower", "flow", "flight"} uses array syntax but is valid but fixed size, not slice. var strs = {"flower", "flow", "flight"} uses invalid braces. strs := string{"flower", "flow", "flight"} uses wrong type syntax.

3. Final Answer:

   strs := []string{"flower", "flow", "flight"} -> Option C

4. Quick Check:

   Slice syntax = []type{values} [OK]

Hint: Use []string{...} to create string slices in Go [OK]

Common Mistakes:

• Using array syntax when slice is needed
• Missing type brackets []
• Using curly braces without type

3. Given a list of strings ["dog", "racecar", "car"], what should the longest common prefix function return and why?

easy

A. "d" because all strings start with 'd'

B. "" because no common prefix exists

C. "c" because 'car' is the shortest string

D. "r" because 'racecar' starts with 'r'

Solution

1. Step 1: Compare prefixes of all strings

   "dog" starts with 'd', "racecar" with 'r', "car" with 'c'. No common starting character exists.

2. Step 2: Determine longest common prefix

   Since no common starting substring exists, the longest common prefix is empty string "".

3. Final Answer:

   "" because no common prefix exists -> Option B

4. Quick Check:

   No common start means empty prefix [OK]

Hint: If first chars differ, prefix is empty [OK]

Common Mistakes:

• Assuming shortest string is prefix
• Picking first string's first letter
• Ignoring all strings must share prefix

4. What is the output of the following Go code that finds the longest common prefix?

package main
import "fmt"
func longestCommonPrefix(strs []string) string {
    if len(strs) == 0 {
        return ""
    }
    prefix := strs[0]
    for _, s := range strs[1:] {
        for len(prefix) > 0 && len(s) >= len(prefix) && s[:len(prefix)] != prefix {
            prefix = prefix[:len(prefix)-1]
        }
    }
    return prefix
}
func main() {
    strs := []string{"inter", "internet", "internal"}
    fmt.Println(longestCommonPrefix(strs))
}

medium

A. int

B. in

C. inte

D. inter

Solution

1. Step 1: Initialize prefix and compare with each string

   Prefix starts as "inter". Compare with "internet": len(s)=8 >=5 and "internet"[:5] == "inter", so no reduction.

2. Step 2: Compare prefix "inter" with "internal"

   len(s)=8 >=5 and "internal"[:5] == "inter", so prefix remains "inter".

3. Final Answer:

   inter -> Option D

4. Quick Check:

   Common start of all strings = "inter" [OK]

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

Common Mistakes:

• Not shrinking prefix correctly
• Comparing prefix with wrong substring
• Returning initial prefix without checks

5. Identify the bug in this Go function for longest common prefix and select the fix:

func longestCommonPrefix(strs []string) string {
    if len(strs) == 0 {
        return ""
    }
    prefix := strs[0]
    for _, s := range strs[1:] {
        for len(prefix) > 0 && s[:len(prefix)] != prefix {
            prefix = prefix[:len(prefix)-1]
        }
    }
    return prefix
}

medium

A. Add check that len(s) >= len(prefix) before s[:len(prefix)]

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

C. Change for loop to iterate over all strings including first

D. Return strs[0] directly without loop

Solution

1. Step 1: Analyze substring slicing in loop

   The code slices s[:len(prefix)] without checking if s is long enough, causing runtime panic if s is shorter than prefix.

2. Step 2: Fix by adding length check

   Adding condition len(s) >= len(prefix) before slicing prevents out-of-range error.

3. Final Answer:

   Add check that len(s) >= len(prefix) before s[:len(prefix)] -> Option A

4. Quick Check:

   Prevent slice out-of-range by length check [OK]

Hint: Always check slice length before slicing [OK]

Common Mistakes:

• Ignoring string length before slicing
• Changing prefix initialization unnecessarily
• Skipping loop over all strings