Here is the solution using backtracking, But, I really think `this problem shouldn't be put under the category of Backtracking, because it got TLE`

:

```
public class Solution {
boolean [] used = new boolean[10];
StringBuilder builder = new StringBuilder();
int count = 0;
public String getPermutation(int n, int k) {
helper(n, k, n);
return builder.toString();
}
public boolean helper(int n, int k, int len){
if(len == 0){
++count;
return count == k? true:false;
}
for(int i = 1;i<=n;++i){
if(used[i]) continue;
used[i] = true;
builder.append(i);
if(helper(n, k, len-1)) return true;
used[i] = false;
builder.deleteCharAt(builder.length()-1);
}
return false;
}
}
```