LeetCode – Secuencia de permutación (Java)

public class Solution {
	public String getPermutation(int n, int k) {
 
		// initialize all numbers
		ArrayList<Integer> numberList = new ArrayList<Integer>();
		for (int i = 1; i <= n; i++) {
			numberList.add(i);
		}
 
		// change k to be index
		k--;
 
		// set factorial of n
		int mod = 1;
		for (int i = 1; i <= n; i++) {
			mod = mod * i;
		}
 
		String result = "";
 
		// find sequence
		for (int i = 0; i < n; i++) {
			mod = mod / (n - i);
			// find the right number(curIndex) of
			int curIndex = k / mod;
			// update k
			k = k % mod;
 
			// get number according to curIndex
			result += numberList.get(curIndex);
			// remove from list
			numberList.remove(curIndex);
		}
 
		return result.toString();
	}
}

public class Solution {
	public String getPermutation(int n, int k) {
		boolean[] output = new boolean[n];
		StringBuilder buf = new StringBuilder("");
 
		int[] res = new int[n];
		res[0] = 1;
 
		for (int i = 1; i < n; i++)
			res[i] = res[i - 1] * i;
 
		for (int i = n - 1; i >= 0; i--) {
			int s = 1;
 
			while (k > res[i]) {
				s++;
				k = k - res[i];
			}
 
			for (int j = 0; j < n; j++) {
				if (j + 1 <= s && output[j]) {
					s++;
				}
			}
 
			output[s - 1] = true;
			buf.append(Integer.toString(s));
		}
 
		return buf.toString();
	}
}