XOR subsequences

You are given an array $A$ consisting of $N$ integers. Your task is to find the longest subsequence S, such that XOR of any two elements in $S$ is non-zero.

You are required to print the size of $S(L)$ and the array of indices that are chosen for $S$. In the case of multiple such arrays, choose the lexicographically smallest array.

Example: Let array A be [7, 9, 7], the maximum length of subsequence can be 2. There are two ways to choose a subsequence, [7, 9] and [9, 7]. The indices selected in the first case are [1, 2] while in the second they are [2, 3] so we need to choose [1, 2] as it is minimal.

Example:

Input:  n = 5, arr = {3, 2, 3, 1, 2}

Output:

3
1 2 4 

Approach:

C++

#include <bits/stdc++.h>
using namespace std;

vector<int> xorSubsequence(int n, vector<int> arr)
{
    set<int> st;
    vector<int> res;
    for (int i = 0; i < n; i++)
    {
        if (st.find(arr[i]) == st.end())
        {
            st.insert(arr[i]);
            res.push_back(i + 1);
        }
    }

    return res;
}
int main()
{

    int n = 5;
    vector<int> arr = {3, 2, 3, 1, 2};

    vector<int> res = xorSubsequence(n, arr);
    cout << res.size() << "\n";

    for (int i = 0; i < res.size(); i++)
        cout << res[i] << " ";
    cout << "\n";

    return 0;
}