用java写一段速度快的排列组合算法

以下是使用Java编写的排列组合算法：

public class CombinationPermutation {

    public static void main(String[] args) {
        int[] arr = {1, 2, 3};
        int r = 2;
        System.out.println("Permutation:");
        permutation(arr, r);
        System.out.println("\nCombination:");
        combination(arr, r);
    }

    private static void permutation(int[] arr, int r) {
        int[] result = new int[r];
        boolean[] used = new boolean[arr.length];
        permutationHelper(arr, result, used, 0);
    }

    private static void permutationHelper(int[] arr, int[] result, boolean[] used, int index) {
        if (index == result.length) {
            System.out.println(Arrays.toString(result));
            return;
        }
        for (int i = 0; i < arr.length; i++) {
            if (!used[i]) {
                result[index] = arr[i];
                used[i] = true;
                permutationHelper(arr, result, used, index + 1);
                used[i] = false;
            }
        }
    }

    private static void combination(int[] arr, int r) {
        int[] result = new int[r];
        combinationHelper(arr, result, 0, 0);
    }

    private static void combinationHelper(int[] arr, int[] result, int index, int start) {
        if (index == result.length) {
            System.out.println(Arrays.toString(result));
            return;
        }
        for (int i = start; i < arr.length; i++) {
            result[index] = arr[i];
            combinationHelper(arr, result, index + 1, i + 1);
        }
    }
}

这里提供了两种方法：排列和组合。`permutation`函数使用回溯算法来生成数组的所有排列，而`combination`函数使用递归算法来生成数组的所有组合。