60. 第k个排列

class Solution {
    int kth ;
    List<Integer> res;
    public String getPermutation(int n, int k) {
        List<Integer> list = new ArrayList<Integer>();
        boolean[] visited = new boolean[n];
        bt(n,k,visited,list);
        String s = "";
        for(int num : res){
            s += num;
        }
        return s;
    }
    private void bt(int n,int k,boolean[] visited,List<Integer> list){
        if(list.size() == n){
            kth++;
            if(kth == k){
                res = new ArrayList<>(list);
                return;
            }
        }
        for(int i = 1;i <= n;i++){
            if(!visited[i - 1]){
                visited[i - 1] = true;
                list.add(i);
                bt(n,k,visited,list);
                list.remove(list.size() - 1);
                visited[i - 1] = false;
            }
        }  
    }
}