All Unique Permutations

• Time : $O(n^2)$
• Space : $O(n)$

void helper(vector<vector<int>>& res, unordered_map<int,int>& freq, int p, int n, vector<int>& now) {
    if(p == n) res.push_back(now);
    else {
        for(auto& [k,v] : freq) {
            if(!v) continue;
            v -= 1;
            now.push_back(k);
            helper(res,freq,p+1,n,now);
            now.pop_back();
            v += 1;
        }
    }
}

vector<vector<int>> Solution::permute(vector<int> &A) {
    unordered_map<int, int> freq;
    for(auto& a : A) freq[a]++;
    vector<vector<int>> res;
    helper(res,freq,0,A.size(), vector<int>() = {});
    return res;
}