18. Count Pairs in an Array

The problem can be found at the following link: Question Link

My Approach

To solve this simple traverse the array, mark elements as negative to indicate their presence.

• If an element is already negative, it means it's a repeated element. Add it to the output vector.

Time and Auxiliary Space Complexity

• Time Complexity: O(n)

• Auxiliary Space Complexity: O(1)

Code (C++)

class Solution {
public:
    vector<int> twoRepeated(int arr[], int n) {
        vector<int> out;
        for (int i = 0; i < n + 2; ++i) {
            if (arr[abs(arr[i])] > 0)
                arr[abs(arr[i])] = -arr[abs(arr[i])];
            else
                out.push_back(abs(arr[i]));
        }
        return out;
    }
};

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