lintcode-medium-Permutation Sequence

class Solution {
    /**
      * @param n: n
      * @param k: the kth permutation
      * @return: return the k-th permutation
      */
    public String getPermutation(int n, int k) {
        if(n <= 0)
            return "";
        
        StringBuilder result = new StringBuilder();
        ArrayList<Integer> list = new ArrayList<Integer>();
        
        for(int i = 1; i <= n; i++)
            list.add(i);
        
        k--;
        n--;
        
        while(n >= 0){
            int index = k / factor(n);
            
            result.append(list.remove(index));
            k -= index * factor(n);
            n--;
        }
        
        return result.toString();
    }
    
    public int factor(int n){
        int result = 1;
        
        for(int i = 1; i <= n; i++)
            result *= i;
        
        return result;
    }
}