DP vs Recursion vs Greedy Choosing the Right Tool in DSA C - Interactive Comparison Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to calculate Fibonacci numbers using simple recursion.

DSA C

int fib(int n) {
    if (n <= 1) {
        return n;
    }
    return fib(n - 1) [1] fib(n - 2);
}

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

A+

B-

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction or multiplication instead of addition.

Forgetting to return the sum.

The Fibonacci number is the sum of the two previous Fibonacci numbers, so we add the results of fib(n-1) and fib(n-2).

2fill in blank

medium

Complete the code to implement memoization in Fibonacci calculation.

DSA C

int fibMemo(int n, int memo[]) {
    if (n <= 1) {
        return n;
    }
    if (memo[n] != -1) {
        return memo[n];
    }
    memo[n] = fibMemo(n - 1, memo) [1] fibMemo(n - 2, memo);
    return memo[n];
}

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

A*

B-

C+

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Not storing the sum in memo array.

Using wrong operators causing incorrect results.

Memoization stores the sum of fib(n-1) and fib(n-2) to avoid repeated calculations.

3fill in blank

hard

Fix the error in the greedy coin change function to return minimum coins needed.

DSA C

int minCoins(int coins[], int n, int amount) {
    int count = 0;
    for (int i = n - 1; i >= 0; i--) {
        while (amount >= coins[i]) {
            amount [1] coins[i];
            count++;
        }
    }
    return amount == 0 ? count : -1;
}

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

A-=

B/=

C*=

D+=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+=' which increases amount causing infinite loop.

Using multiplication or division operators incorrectly.

To reduce the amount by the coin value, we subtract coins[i] from amount using '-=' operator.

4fill in blank

hard

Fill both blanks to implement bottom-up DP for Fibonacci numbers.

DSA C

int fibDP(int n) {
    int dp[n + 1];
    dp[0] = 0;
    dp[1] = 1;
    for (int i = 2; i <= n; i++) {
        dp[i] = dp[i [1] 1] [2] dp[i - 2];
    }
    return dp[n];
}

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

A-

B+

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong operators causing index errors.

Mixing multiplication or division instead of addition.

We calculate dp[i] by adding dp[i-1] and dp[i-2]. The first blank is '-' to access previous index, second blank is '+' to add values.

5fill in blank

hard

Fill all three blanks to create a greedy algorithm for activity selection problem.

DSA C

void selectActivities(int start[], int finish[], int n) {
    int i = 0;
    printf("Activity %d selected\n", i);
    for (int j = 1; j < n; j++) {
        if (start[j] [1] finish[[3]]) {
            printf("Activity %d selected\n", j);
            i = [2];
        }
    }
}

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

A>=

Bj

C<=

Di

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong comparison operators causing wrong selections.

Not updating the last selected activity index correctly.

We select activity j if its start time is greater or equal to finish time of last selected activity i. Then update i to j.