Recursion on Arrays and Strings in DSA Go - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to return the sum of elements in the array recursively.

DSA Go

func sumArray(arr []int, n int) int {
    if n == 0 {
        return 0
    }
    return arr[n-1] + [1]
}

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

AsumArray(arr, n)

BsumArray(arr, n-1)

CsumArray(arr, n+1)

DsumArray(arr, 0)

Attempts:

3 left

💡 Hint

Common Mistakes

Using n+1 causes infinite recursion.

Calling sumArray with n does not reduce the problem size.

The function calls itself with n-1 to sum all elements up to index n-1 recursively.

2fill in blank

medium

Complete the code to check if a string is a palindrome using recursion.

DSA Go

func isPalindrome(s string, start int, end int) bool {
    if start >= end {
        return true
    }
    if s[start] != s[end] {
        return false
    }
    return [1]
}

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AisPalindrome(s, start, end-1)

BisPalindrome(s, start-1, end+1)

CisPalindrome(s, start+1, end-1)

DisPalindrome(s, start+1, end)

Attempts:

3 left

💡 Hint

Common Mistakes

Moving indices in the wrong direction causes incorrect checks.

Not moving both indices causes infinite recursion.

The function moves inward by increasing start and decreasing end to check characters recursively.

3fill in blank

hard

Fix the error in the recursive function to find the maximum element in an array.

DSA Go

func maxElement(arr []int, n int) int {
    if n == 1 {
        return arr[0]
    }
    maxRest := maxElement(arr, [1])
    if arr[n-1] > maxRest {
        return arr[n-1]
    }
    return maxRest
}

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

An-1

Bn+1

C0

Dn

Attempts:

3 left

💡 Hint

Common Mistakes

Using n+1 causes index out of range error.

Using n causes infinite recursion.

The function reduces the problem size by calling itself with n-1 to find max in smaller array.

4fill in blank

hard

Fill both blanks to create a recursive function that counts occurrences of a character in a string.

DSA Go

func countChar(s string, c byte, index int) int {
    if index == [1] {
        return 0
    }
    count := 0
    if s[index] == [2] {
        count = 1
    }
    return count + countChar(s, c, index+1)
}

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

Alen(s)

Bc

Cindex

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using index instead of len(s) causes infinite recursion.

Comparing with wrong variable causes incorrect count.

The base case is when index reaches string length; compare s[index] with character c.

5fill in blank

hard

Fill both blanks to implement a recursive function that reverses a string.

DSA Go

func reverseString(s string) string {
    if len(s) == 0 {
        return ""
    }
    return string(s[len(s)-1]) + [1](s[2]1)
}

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

AreverseString

B:

C[:len(s)-

D]

Attempts:

3 left

💡 Hint

Common Mistakes

Incorrect slicing syntax causes compile errors.

Not calling reverseString recursively causes infinite recursion.

The function takes last character and adds reversed substring excluding last character.

Practice

1. What does a recursive function do when processing an array?

easy

A. It only processes the first element and stops

B. It loops through the array without calling itself

C. It creates a new array each time without recursion

D. It calls itself with a smaller part of the array until a base case is reached

Solution

1. Step 1: Understand recursion on arrays

   A recursive function processes an array by calling itself with a smaller portion, usually by reducing the size or moving the index.

2. Step 2: Identify the base case

   The recursion stops when the array is empty or the index reaches the end, preventing infinite calls.

3. Final Answer:

   It calls itself with a smaller part of the array until a base case is reached -> Option D

4. Quick Check:

   Recursion = smaller calls until base case [OK]

Hint: Recursion breaks problem into smaller parts until simple case [OK]

Common Mistakes:

• Thinking recursion loops without reducing problem size
• Ignoring the base case leading to infinite recursion
• Confusing recursion with iteration

2. Which of the following is the correct syntax to define a recursive function in Go that processes a string?

easy

A. func recurse(s string) string { if len(s) == 0 { return "" } return string(s[0]) + recurse(s[1:]) }

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

C. func recurse(s string) { if len(s) == 0 { return } else { recurse(s) } }

D. func recurse(s string) string { return s + recurse(s) }

Solution

1. Step 1: Check function signature and base case

   func recurse(s string) string { if len(s) == 0 { return "" } return string(s[0]) + recurse(s[1:]) } defines a function with a base case checking if string length is zero, returning empty string.

2. Step 2: Confirm recursive call reduces problem size

   func recurse(s string) string { if len(s) == 0 { return "" } return string(s[0]) + recurse(s[1:]) } calls itself with substring s[1:], reducing string size each call, correctly building result.

3. Final Answer:

   func recurse(s string) string { if len(s) == 0 { return "" } return string(s[0]) + recurse(s[1:]) } -> Option A

4. Quick Check:

   Base case + smaller input = correct recursion [OK]

Hint: Base case + smaller input slice = correct recursion syntax [OK]

Common Mistakes:

• Missing base case causing infinite recursion
• Not reducing string size in recursive call
• Incorrect return types or missing return

3. What is the output of the following Go code?

package main
import "fmt"

func sum(arr []int) int {
    if len(arr) == 0 {
        return 0
    }
    return arr[0] + sum(arr[1:])
}

func main() {
    result := sum([]int{1, 2, 3, 4})
    fmt.Println(result)
}

medium

A. 10

B. 1234

C. 0

D. Error

Solution

1. Step 1: Understand the recursive sum function

   The function sums the first element and calls itself with the rest of the array until empty.

2. Step 2: Calculate sum step-by-step

   sum([1,2,3,4]) = 1 + sum([2,3,4]) = 1 + 2 + sum([3,4]) = 1 + 2 + 3 + sum([4]) = 1 + 2 + 3 + 4 + sum([]) = 10

3. Final Answer:

   10 -> Option A

4. Quick Check:

   Sum of 1+2+3+4 = 10 [OK]

Hint: Sum adds first element + recursive sum of rest [OK]

Common Mistakes:

• Confusing concatenation with addition
• Forgetting base case returns 0
• Misreading slice syntax arr[1:]

4. Identify the error in this recursive Go function that reverses a string:

package main
import "fmt"

func reverse(s string) string {
    if len(s) == 0 {
        return ""
    }
    return reverse(s[1:]) + s[0]
}

func main() {
    fmt.Println(reverse(123))
}

medium

A. Missing base case in recursion

B. Passing an integer instead of a string to reverse

C. Incorrect string concatenation order

D. Infinite recursion due to no slice reduction

Solution

1. Step 1: Check function call in main

   The function reverse expects a string, but main calls reverse(123), an integer, causing a type error.

2. Step 2: Confirm recursion and base case are correct

   The function has a proper base case and reduces string size correctly, so no recursion error.

3. Final Answer:

   Passing an integer instead of a string to reverse -> Option B

4. Quick Check:

   Type mismatch in function argument [OK]

Hint: Check argument types match function parameters [OK]

Common Mistakes:

• Ignoring type mismatch errors
• Assuming recursion error without checking call
• Confusing string indexing with integer

5. Given a recursive function that removes all vowels from a string, which approach correctly applies recursion on strings in Go?

hard

A. Replace vowels with empty string using built-in functions only

B. Loop through string and remove vowels without recursion

C. Check first character; if vowel, skip and recurse on rest; else include and recurse

D. Call recursion without reducing string size causing infinite loop

Solution

1. Step 1: Understand recursive vowel removal

   The function should check the first character, decide to keep or skip it, then recurse on the rest of the string.

2. Step 2: Confirm recursion reduces problem size and builds result

   By slicing the string smaller each call and concatenating results, recursion correctly removes vowels.

3. Final Answer:

   Check first character; if vowel, skip and recurse on rest; else include and recurse -> Option C

4. Quick Check:

   Recursion with condition + smaller input = correct approach [OK]

Hint: Use condition + smaller substring in recursion [OK]

Common Mistakes:

• Not reducing string size causing infinite recursion
• Using loops instead of recursion
• Ignoring condition to skip vowels