First and Last Occurrence of Element in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find the first occurrence of 'target' in the array.

DSA C++

int firstOccurrence(vector<int>& arr, int target) {
    for (int i = 0; i < arr.size(); i++) {
        if (arr[i] == [1]) {
            return i;
        }
    }
    return -1;
}

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Aarr[i]

Btarget

Ci

Darr.size()

Attempts:

3 left

💡 Hint

Common Mistakes

Using arr.size() instead of target in the if condition.

Comparing i with target instead of arr[i].

We compare each element arr[i] with the target to find the first occurrence.

2fill in blank

medium

Complete the code to find the last occurrence of 'target' in the array.

DSA C++

int lastOccurrence(vector<int>& arr, int target) {
    for (int i = arr.size() - 1; i >= 0; i--) {
        if (arr[i] == [1]) {
            return i;
        }
    }
    return -1;
}

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Aarr.size()

Bi

Ctarget

Darr[i]

Attempts:

3 left

💡 Hint

Common Mistakes

Using i instead of target in the if condition.

Comparing arr.size() with arr[i].

We compare each element arr[i] with the target starting from the end to find the last occurrence.

3fill in blank

hard

Fix the error in the binary search to find the first occurrence of 'target'.

DSA C++

int firstOccurrenceBinarySearch(vector<int>& arr, int target) {
    int low = 0, high = arr.size() - 1, result = -1;
    while (low <= high) {
        int mid = low + (high - low) / 2;
        if (arr[mid] == [1]) {
            result = mid;
            high = mid - 1;
        } else if (arr[mid] < target) {
            low = mid + 1;
        } else {
            high = mid - 1;
        }
    }
    return result;
}

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Atarget

Bmid

Clow

Dhigh

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing arr[mid] with mid instead of target.

Updating wrong pointers after comparison.

We compare arr[mid] with the target to find the first occurrence using binary search.

4fill in blank

hard

Fill both blanks to complete the binary search for the last occurrence of 'target'.

DSA C++

int lastOccurrenceBinarySearch(vector<int>& arr, int target) {
    int low = 0, high = arr.size() - 1, result = -1;
    while (low <= high) {
        int mid = low + (high - low) / 2;
        if (arr[mid] == [1]) {
            result = mid;
            low = mid [2] 1;
        } else if (arr[mid] < target) {
            low = mid + 1;
        } else {
            high = mid - 1;
        }
    }
    return result;
}

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Atarget

Bhigh

C+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' instead of '+' when updating low pointer.

Comparing arr[mid] with high instead of target.

When arr[mid] equals target, update result and move low pointer to mid + 1 to find last occurrence.

5fill in blank

hard

Fill all three blanks to create a function that returns both first and last occurrences of 'target'.

DSA C++

pair<int, int> findFirstAndLast(vector<int>& arr, int target) {
    int first = -1, last = -1;
    int low = 0, high = arr.size() - 1;
    while (low <= high) {
        int mid = low + (high - low) / 2;
        if (arr[mid] == [1]) {
            first = mid;
            high = mid [2] 1;
        } else if (arr[mid] < target) {
            low = mid + 1;
        } else {
            high = mid - 1;
        }
    }
    low = 0; high = arr.size() - 1;
    while (low <= high) {
        int mid = low + (high - low) / 2;
        if (arr[mid] == [3]) {
            last = mid;
            low = mid + 1;
        } else if (arr[mid] < target) {
            low = mid + 1;
        } else {
            high = mid - 1;
        }
    }
    return {first, last};
}

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Atarget

B-

C+

Dmid

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of '-' when updating high pointer in first search.

Mixing up target and mid in comparisons.

Use binary search twice: first to find first occurrence (move high to mid - 1), then last occurrence (move low to mid + 1).