leetcode - permutation sequence

gistfile1.java

    public String getPermutation(int n, int k) {
        if (n <= 0 || k <= 0)
            return "";
        // can't use array because we need remove item each time
        // create the number array
        int mod = 1;
        ArrayList<Integer> nl = new ArrayList<Integer>();
        for (int i = 1; i <= n; i++) {
            nl.add(i);
            mod *= i; // calculate n!
        }
        k--; // 0-based
        
        StringBuilder sb = new StringBuilder();
        
        for (int i = 0; i < n; i++) {
            mod /= (n - i); //start from (n-1)!
            int idx = k / mod; // the root of subtree
            k %= mod;
            sb.append(nl.get(idx));
            nl.remove(idx);
        }
        return sb.toString();
    }