Permutation Sequence

找出规律

public class Solution {
    public String getPermutation(int n, int k) {
        int total = 1;
        ArrayList<Integer> rest = new ArrayList<Integer>();
        StringBuilder result = new StringBuilder();
        int m = n;
        
        while(m > 1){
            total*=m;
            m--;
        }
        
        for(int i = 1; i <= n; i++)
            rest.add(i);
            
        k--;
        while(!rest.isEmpty()){
            total /= n;
            n--;
            int index = k / total;
            result.append(String.valueOf(rest.get(index)));
            k %= total;
            rest.remove(index);
        }
        return result.toString();
    }
}