String Reversal Approaches in DSA Go - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to reverse a string using a for loop.

DSA Go

func reverseString(s string) string {
    result := ""
    for i := len(s) - 1; i >= [1]; i-- {
        result += string(s[i])
    }
    return result
}

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

A1

Blen(s)

C0

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 instead of 0 as the loop boundary causes the first character to be skipped.

Using len(s) causes index out of range error.

The loop should start from the last index (len(s)-1) and go down to 0 to include the first character.

2fill in blank

medium

Complete the code to reverse a string by converting it to a rune slice.

DSA Go

func reverseString(s string) string {
    runes := []rune(s)
    for i, j := 0, len(runes)-1; i < j; i, j = i+1, [1] {
        runes[i], runes[j] = runes[j], runes[i]
    }
    return string(runes)
}

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

Aj-1

Bj+1

Ci-1

Dj

Attempts:

3 left

💡 Hint

Common Mistakes

Incrementing j causes the loop to never end or swap incorrectly.

Using j without change causes an infinite loop.

To swap characters from both ends, j should decrease by 1 each iteration.

3fill in blank

hard

Fix the error in the code that attempts to reverse a string using a byte slice.

DSA Go

func reverseString(s string) string {
    bytes := []byte(s)
    for i := 0; i < len(bytes)/2; i++ {
        j := len(bytes) - i - [1]
        bytes[i], bytes[j] = bytes[j], bytes[i]
    }
    return string(bytes)
}

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

A0

B1

C2

Di

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 causes swapping with an invalid index.

Using i or 2 causes wrong indices and runtime errors.

To get the correct index from the end, subtract i and 1 from length.

4fill in blank

hard

Fill both blanks to create a map of characters to their reversed positions.

DSA Go

func charPositions(s string) map[rune]int {
    positions := make(map[rune]int)
    for i, r := range s {
        positions[r] = [1] - i - [2]
    }
    return positions
}

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

Alen(s)

Blen(s) - 1

Ci

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using len(s) instead of len(s)-1 causes off-by-one errors.

Subtracting i incorrectly shifts positions.

The reversed position is length minus 1 minus current index i, so subtract 0 after.

5fill in blank

hard

Fill both blanks to create a reversed string using a builder and rune slice.

DSA Go

import "strings"

func reverseString(s string) string {
    runes := []rune(s)
    var builder strings.Builder
    for i := len(runes) - 1; i >= [1]; i-- {
        builder.WriteRune(runes[[2]])
    }
    return builder.String()
}

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

A0

Bi

Clen(runes) - 1

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Looping down to 1 skips the first character.

Writing runes at wrong index causes incorrect output.

Loop down to 0, write runes at index i, and return the built string.

Practice

1. What does the strings.Reverse function do in Go if it existed? (Assume it reverses the string.)

easy

A. It converts the string to uppercase.

B. It returns the input string with characters in reverse order.

C. It removes all vowels from the string.

D. It splits the string into words.

Solution

1. Step 1: Understand the function name and purpose

   The function name Reverse suggests it changes the order of characters.

2. Step 2: Interpret the expected behavior

   Reversing a string means the last character becomes first, and so on.

3. Final Answer:

   It returns the input string with characters in reverse order. -> Option B

4. Quick Check:

   Reverse means flip order [OK]

Hint: Reverse means flip the order of characters [OK]

Common Mistakes:

• Confusing reverse with uppercase conversion
• Thinking reverse removes characters
• Assuming reverse splits words

2. Which of the following Go code snippets correctly reverses a string s?

easy

A. var reversed string
for i := len(s)-1; i >= 0; i-- {
reversed += string(s[i])
}

B. for i := 0; i < len(s); i++ {
reversed += string(s[i])
}

C. reversed := strings.ToUpper(s)

D. reversed := strings.Split(s, " ")

Solution

1. Step 1: Analyze the loop direction

   var reversed string for i := len(s)-1; i >= 0; i-- { reversed += string(s[i]) } loops from the last character to the first, building reversed string.

2. Step 2: Check other options

   for i := 0; i < len(s); i++ { reversed += string(s[i]) } loops forward, so no reversal. reversed := strings.ToUpper(s) changes case, reversed := strings.Split(s, " ") splits words.

3. Final Answer:

   Option A correctly reverses the string. -> Option A

4. Quick Check:

   Loop backward to reverse [OK]

Hint: Loop backward from end to start to reverse [OK]

Common Mistakes:

• Looping forward instead of backward
• Using string functions that don't reverse
• Confusing splitting with reversing

3. What is the output of this Go code?

func main() {
  s := "hello"
  reversed := ""
  for i := len(s) - 1; i >= 0; i-- {
    reversed += string(s[i])
  }
  fmt.Println(reversed)
}

medium

A. olleh

B. hello

C. h

D. error

Solution

1. Step 1: Trace the loop from end to start

   The loop starts at index 4 ('o') and goes to 0 ('h'), appending characters in reverse order.

2. Step 2: Build the reversed string

   Characters appended: 'o', 'l', 'l', 'e', 'h' forming "olleh".

3. Final Answer:

   olleh -> Option A

4. Quick Check:

   Reverse of "hello" is "olleh" [OK]

Hint: Reverse string by appending chars from end to start [OK]

Common Mistakes:

• Reading string forward instead of backward
• Printing original string by mistake
• Expecting error due to string indexing

4. Find the error in this Go code that tries to reverse a string s:

func reverse(s string) string {
  var reversed string
  for i := 0; i <= len(s); i++ {
    reversed += string(s[len(s)-i])
  }
  return reversed
}

medium

A. The function should return reversed as a byte slice.

B. The string concatenation should use +=.

C. The loop condition should be i < len(s), not <=.

D. The loop should start from len(s) and go down to 0.

Solution

1. Step 1: Check loop bounds

   The loop runs from 0 to len(s) inclusive, but string indices go from 0 to len(s)-1.

2. Step 2: Identify out-of-range index

   Accessing s[len(s)-i] when i=0 is s[len(s)], which is out of range and causes runtime error.

3. Final Answer:

   The loop condition should be i < len(s), not <=. -> Option C

4. Quick Check:

   Index must be within 0 to len(s)-1 [OK]

Hint: Loop index must stay inside string length bounds [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Accessing index len(s) which is invalid
• Confusing string and byte slice types

5. Given a string with Unicode characters like "Go语言", which approach correctly reverses it preserving characters properly in Go?

hard

A. Split string by spaces and reverse the words.

B. Use a byte slice and reverse bytes directly.

C. Use strings.ToUpper and then reverse.

D. Convert string to []rune, reverse the rune slice, then convert back to string.

Solution

1. Step 1: Understand Unicode in Go strings

   Go strings are byte sequences; Unicode characters may be multiple bytes.

2. Step 2: Use rune slice to handle Unicode characters

   Converting to []rune treats each Unicode character as one element, safe to reverse.

3. Step 3: Reverse rune slice and convert back

   Reversing []rune reverses characters correctly, then convert back to string.

4. Final Answer:

   Convert string to []rune, reverse runes, then back to string. -> Option D

5. Quick Check:

   Use runes for Unicode-safe reversal [OK]

Hint: Use []rune to reverse Unicode strings safely [OK]

Common Mistakes:

• Reversing bytes breaks Unicode characters
• Using uppercase conversion instead of reversing
• Reversing words instead of characters