java递归打印相反

Question

I am using recursion in Java, the output should look like

###
##*
#**
***

This is how I'm doing it right now:

public void recursion(int n){
    if (n==0) {
        return;
    }
    else {
        System.out.print(number_pound(n));
        System.out.println();

        recursion(n-1);
        System.out.print(stars(n)); 
        System.out.println();

    }
}

private String number_pound(int level) {
    String s = "";
    for(int i=0;i<level;i++)
        s+="#";
    return s;
}

private String stars(int level) {
    String s = "";
    for (int i = 0; i < level; i++)
        s+= "*";
    return s;
}

public static void main(String[] args) {
    recursion rec = new recursion();
    rec.recursion(3);
}

which gives me the result in this way:

###
##
#
*
**
***

I know that the problem with my code is that I have the System.out.println()to separate two parts, but without the printline statement, the number pound will be jammed in the same line.

Answer 1

This will do what you are trying to do, I believe. It will print the number of pounds, then the number of stars, and then the new line. You will have to pass an additional length parameter, since you need to specify the length of the output string. You cannot do without the length, I believe.

public void recursion(int n, int length){
    if (n<0) {
        return;
    }
    else {
        System.out.print(number_pound(n));
        System.out.print(stars(length - n)); 
        System.out.println();
        recursion(n-1, length);
    }
}

So, now your call in the main looks like rec.recursion(3, 3)

Moreover I believe it would be much better to do it iteratively than using recursion.

Answer 2

You have missed few things:

• You first want to print pounds, then starts, then new-line and then repeat the process
• The number of stars printed on each line is actually "the initial number passed - level"
• You need these two numbers in recursion()
• You want to repeat the process even when n is 0 .

This is how it should be:

    public void recursion(int max, int n){
        if (n<0) {
            return;
        }
        else {
            System.out.print(number_pound(n));
            System.out.print(stars(max - n)); 
            System.out.println();
            recursion(max, n-1);

        }
    }

    private String number_pound(int level) {
        String s = "";
        for(int i=0;i<level;i++)
            s+="#";
        return s;
    }

private String stars(int level) {
    String s = "";
    for (int i = 0; i <level; i++)
        s+= "*";
    return s;
}

public static void main(String[] args) {
    recursion rec = new recursion();
    rec.recursion(3, 3);
}

Couple of additions:

• Use naming standards. Class-names should start with capital letter recursion is not a good name.
• Also it is bad practice to have class and method with exactly same name. So consider renaming class recursion .

Answer 3

public class recursion {
     int k;                 
    public void recursion(int n){

      if(k==0){
        k=n;
       }
    if (n<0) {
        return;
    }
    else {

        System.out.print(number_pound(n)+stars(k-n));
        System.out.println();

        recursion(n-1);


    }
}

private String number_pound(int level) {
    String s = "";
    for(int i=0;i<level;i++)
        s+="#";
    return s;
}

private String stars(int level) {
    String s = "";
    for (int i = 0; i < level; i++)
        s+= "*";
    return s;
}

public static void main(String[] args) {
    recursion rec = new recursion();

    rec.recursion(3);
}

}

Answer 4

A variable length should represent the max number (3 in your case). You just need to keep displaying n #s and length - n stars until n becomes less than 0.

public void recursion(int n, int length){
    if(n>=0){
        System.out.print(number_pound(n));
        System.out.print(stars(length - n)+"\n");
        recursion(n-1, length);
    }
}

Call recursion(3,3) to get the output. If you want the signature of the recursion() method to be (int n) only, then you could use a global static variable to refer to the max number to which you assign the max number in the main method before calling recursion().

Answer 5

int length = 3;

private List<String> newList(String x, int length) {
    return Arrays.asList(new String[length]).stream().map(s -> x)
           .collect(Collectors.toList());
}

private void recurSymbol(String x, String y, int length) {
    List<String> list = newList(x, length);
    list.addAll(newList(y, this.length - length));
    System.out.println(String.join("", list));
    if(length > 0) recurSymbol(x, y, length - 1);
}

public void recursion(String x, String y) {
    recurSymbol(x, y, length);
}

Then just call something like

recursion("#", "*");

Obviously, length can be any int 0 or greater.