Recursion on Arrays and Strings in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA C

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

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AsumArray(arr, n + 1)

BsumArray(arr, n - 1)

CsumArray(arr, n)

DsumArray(arr, 0)

Attempts:

3 left

💡 Hint

Common Mistakes

Using n+1 causes infinite recursion.

Using n causes the same call repeatedly.

Returning arr[n] causes out-of-bounds access.

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 C

int isPalindrome(char str[], int start, int end) {
    if (start >= end) {
        return 1;
    }
    if (str[start] != str[end]) {
        return 0;
    }
    return [1];
}

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AisPalindrome(str, start + 1, end - 1)

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

CisPalindrome(str, start, end)

DisPalindrome(str, end, start)

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping start and end causes wrong checks.

Using same start and end causes infinite recursion.

Decreasing start or increasing end moves away from center.

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 count occurrences of a character in a string.

DSA C

int countChar(char str[], int index, char c) {
    if (str[index] == '\0') {
        return 0;
    }
    int count = (str[index] == c) ? 1 : 0;
    return count + [1];
}

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AcountChar(str, 0, c)

BcountChar(str, index - 1, c)

CcountChar(str, index, c)

DcountChar(str, index + 1, c)

Attempts:

3 left

💡 Hint

Common Mistakes

Decreasing index causes infinite recursion or invalid access.

Using same index causes infinite recursion.

Resetting index to 0 causes repeated counting.

The function should move forward by increasing index to check next character recursively.

4fill in blank

hard

Fill both blanks to create a recursive function that reverses a string in place.

DSA C

void reverseString(char str[], int start, int end) {
    if (start >= end) {
        return;
    }
    char temp = str[[1]];
    str[[2]] = str[start];
    str[start] = temp;
    reverseString(str, start + 1, end - 1);
}

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Aend

Bstart

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping with wrong indices causes incorrect reversal.

Using same index twice causes no change.

Not moving indices inward causes infinite recursion.

Swap characters at start and end indices, then recurse inward.

5fill in blank

hard

Fill all three blanks to create a recursive function that builds a frequency map of characters in a string.

DSA C

void buildFreqMap(char str[], int index, int freq[]) {
    if (str[[1]] == '\0') {
        return;
    }
    freq[str[[2]] - 'a']++;
    buildFreqMap(str, [3], freq);
}

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Aindex

Cindex + 1

Dindex - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong index causes incorrect frequency counting.

Decreasing index causes infinite recursion.

Not checking for string end causes errors.

Use index to check current character and increment frequency, then recurse with index + 1.