Program66.java

public class Program66 {

    //Move all'x' to the end of the string -> "axbcxxd"
    public static void moveAllX(String str, int idx, int count, String newString) {
        if (idx == str.length()) {
            for (int i = 0; i < count; i++) {
                newString += 'x';
            }
            System.out.println(newString);
            return;

        }

        char currChar = str.charAt(idx);
        if (currChar == 'x') {
            count++;
            moveAllX(str, idx + 1, count, newString);
        } else {
            newString += currChar;//newString = newString+currChar
            moveAllX(str, idx + 1, count, newString);
        }
    }

    public static void main(String[] args) {
        String str = "axbcxxd";
        moveAllX(str, 0, 0, "");
    }

}
// time complexity : traversing= O(n) + recursive calls O(n)-> O(2n) = O(n) [in assymptotic notation we remove the constant factor]