Permutations Backtracking

def permute(nums):
    res=[]
    def bt(path,used):
        if len(path)==len(nums): res.append(path[:]); return
        for i in range(len(nums)):
            if used[i]: continue
            used[i]=True; path.append(nums[i])
            bt(path,used)
            used[i]=False; path.pop()
    bt([],[False]*len(nums))
    return res

public List<List<Integer>> permute(int[] nums){
    List<List<Integer>> res=new ArrayList<>();
    backtrack(res,new ArrayList<>(),nums,new boolean[nums.length]);
    return res;
}
void backtrack(List<List<Integer>> res,List<Integer> path,int[] nums,boolean[] used){
    if(path.size()==nums.length){res.add(new ArrayList<>(path));return;}
    for(int i=0;i<nums.length;i++){
        if(used[i]) continue;
        used[i]=true; path.add(nums[i]);
        backtrack(res,path,nums,used);
        used[i]=false; path.remove(path.size()-1);
    }
}