What if the input is like this 1 10 6 1 4 5 5 6 2 which contain two solutions do we give both or we just give the first solution.

public static List<Integer> icecreamParlor(int m, List<Integer> arr) {
    List<Integer> result = new ArrayList<>();
    for(int i=0;i<arr.size()-1;i++) {
        int diff = m-arr.get(i);
        int idx = arr.indexOf(diff);
        if(idx == i) {
            idx = arr.subList(i+1, arr.size()).indexOf(diff);
            if(idx != -1) idx = i+1+idx;
        }
        if(idx == -1) continue;

        result.add(i+1);
        result.add(idx+1);
        return result;
    }
    return result;
}

There is inconsitent use of variable name, early on the question it named as m later on the passage, it is named as . Please be consistent.

Here is my c++ solution, you can watch the explanation here : https://youtu.be/WvLuA-LCsb0

vector<int> icecreamParlor(int m, vector<int> arr) {
    map<int, int> mp;
    for (int i = 0; i < arr.size(); i++) {
        int complement = m - arr[i];
        if (mp.count(complement)) {
            return {mp[complement] + 1, i + 1};
        }
        mp[arr[i]] = i;
    }
    return {};
}