Example: MissingElementsHashing in C++

// MissingElementsHashing
#include <iostream>
#include <vector>
#include <unordered_set> // For std::unordered_set

using namespace std;

// Function to find missing elements using an unordered_set
vector<int> findMissingHashing(const vector<int>& arr, int lowerBound, int upperBound) {
    vector<int> missingElements;
    if (lowerBound > upperBound) {
        return {}; // Invalid range
    }

    // Step 1: Insert all elements of the input array into an unordered_set
    unordered_set<int> presentNumbers;
    for (int num : arr) {
        // Only insert numbers that could potentially be in the range
        // This is an optimization; inserting all won't break correctness.
        if (num >= lowerBound && num <= upperBound) {
            presentNumbers.insert(num);
        }
    }

    // Step 2: Iterate through the range and check for missing numbers
    for (int i = lowerBound; i <= upperBound; ++i) {
        if (presentNumbers.find(i) == presentNumbers.end()) {
            // If 'i' is not found in the set, it's missing
            missingElements.push_back(i);
        }
    }

    return missingElements;
}

int main() {
    // Step 1: Define the input array and the range
    vector<int> numbers = {10, 12, 11, 15, 8, 17};
    int lower = 10;
    int upper = 15;

    cout << "Input array: ";
    for (int num : numbers) {
        cout << num << " ";
    }
    cout << endl;
    cout << "Range: [" << lower << ", " << upper << "]" << endl;

    // Step 2: Call the function to find missing elements
    vector<int> missing = findMissingHashing(numbers, lower, upper);

    // Step 3: Display the results
    cout << "Missing elements in range [" << lower << ", " << upper << "]: ";
    if (missing.empty()) {
        cout << "None";
    } else {
        for (int num : missing) {
            cout << num << " ";
        }
    }
    cout << endl;

    // Example with a different array
    vector<int> nums2 = {1, 3, 5, 7, 9};
    int lower2 = 1;
    int upper2 = 10;
    cout << "\nInput array: ";
    for (int num : nums2) {
        cout << num << " ";
    }
    cout << endl;
    cout << "Range: [" << lower2 << ", " << upper2 << "]" << endl;
    vector<int> missing2 = findMissingHashing(nums2, lower2, upper2);
    cout << "Missing elements in range [" << lower2 << ", " << upper2 << "]: ";
    if (missing2.empty()) {
        cout << "None";
    } else {
        for (int num : missing2) {
            cout << num << " ";
        }
    }
    cout << endl;

    return 0;
}