Permutation of a Integer in descending order

Question

I am trying to return the permutations of an integer in descending order. Currently I have code which returns all the permutations of a number however, it is not in descending order. I am unsure to whether I should use an array instead and it may be easier. My current code is:

// Function to print all the permutations of str
static void printPermutn(String str, String ans)
{
    // If string is empty
    if (str.length() == 0) {
        System.out.print(ans + " ");
        return;
    }

    for (int i = 0; i < str.length(); i++) {

        // 1st character of str
        char ch = str.charAt(i);

        // Rest of the string after excluding
        // the 1st character
        String ros = str.substring(0, i) +
                str.substring(i + 1);

        // Recurvise call

        printPermutn(ros, ans + ch);
    }
}

since you mentioned integer en.wikipedia.org/wiki/Heap%27s_algorithm — karthick
– karthick, Commented Oct 8, 2019 at 20:58

Ahmed Nabil · Accepted Answer · 2019-10-08 20:53:44Z

You can use global List<String> permutations then put all values into this collection
Finally, you can sort it in a descending order using
Collections.sort(permutations, Collections.reverseOrder());

private static List<String> permutations;

public static void main(String[] args) {
    permutations = new ArrayList<>();
    printPermutn("123", "");

    System.out.println();
    System.out.println("permutations BEFORE sorting");
    System.out.println(permutations);

    // sort
    Collections.sort(permutations, Collections.reverseOrder());

    System.out.println("permutations AFTER sorting");
    System.out.println(permutations);
}

// Function to print all the permutations of str
static void printPermutn(String str, String ans) {
    // If string is empty
    if (str.length() == 0) {
        System.out.print(ans + " ");
        permutations.add(ans);
        return;
    }

    for (int i = 0; i < str.length(); i++) {

        // 1st character of str
        char ch = str.charAt(i);

        // Rest of the string after excluding
        // the 1st character
        String ros = str.substring(0, i) + str.substring(i + 1);

        // Recurvise call

        printPermutn(ros, ans + ch);
    }
}

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