Two Sum Problem Classic Hash Solution in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the hash map for storing numbers and their indices.

DSA C

int hashMap[[1]];

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A100

B1000

C10

D100000

Attempts:

3 left

💡 Hint

Common Mistakes

Using too small a size causing collisions or out-of-bound errors.

Using an uninitialized or zero-sized array.

The hash map size is set to 100000 to cover the range of possible input values efficiently.

2fill in blank

medium

Complete the code to initialize the hash map values to -1 indicating empty slots.

DSA C

for (int i = 0; i < [1]; i++) {
    hashMap[i] = -1;
}

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

A100000

B1000

C100

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing fewer elements than the hash map size.

Not initializing the hash map at all.

The loop runs up to 100000 to initialize all hash map entries to -1.

3fill in blank

hard

Fix the error in the condition to check if the complement exists in the hash map.

DSA C

if (hashMap[[1]] != -1) {
    return (int[]){hashMap[complement], i};
}

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Acomplement

Bnums[i]

Ci

Dtarget

Attempts:

3 left

💡 Hint

Common Mistakes

Checking hashMap[nums[i]] instead of hashMap[complement].

Using the wrong variable in the condition.

The complement is the key to check in the hash map to find if the pair exists.

4fill in blank

hard

Fill both blanks to store the current index in the hash map and calculate the complement.

DSA C

int complement = [1] - nums[i];
hashMap[[2]] = i;

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Atarget

Bnums[i]

Ci

Dcomplement

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping complement and nums[i] in assignments.

Storing complement instead of nums[i] in hash map.

The complement is target minus current number, and we store the current index at nums[i] in the hash map.

5fill in blank

hard

Fill all three blanks to complete the Two Sum function returning indices of the two numbers.

DSA C

int* twoSum(int* nums, int numsSize, int target, int* returnSize) {
    static int hashMap[100000];
    for (int i = 0; i < [1]; i++) {
        hashMap[i] = -1;
    }
    for (int i = 0; i < numsSize; i++) {
        int complement = [2] - nums[i];
        if (hashMap[complement] != -1) {
            int* result = malloc(2 * sizeof(int));
            result[0] = hashMap[complement];
            result[1] = i;
            *returnSize = 2;
            return result;
        }
        hashMap[[3]] = i;
    }
    *returnSize = 0;
    return NULL;
}

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

A100000

Btarget

Cnums[i]

Di

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong loop size for initialization.

Mixing up complement and nums[i] in hash map access.

Not allocating memory for the result array.

The hash map is initialized with size 100000. The complement is target - nums[i]. We store the index i at hashMap[nums[i]].