编号吡喃酰胺，采用递归方法。 Java初学者

Question

I am a new member here, and I am also a beginner in JAVA. The thing that seems the most abstract to me is recursion, and so I have some difficulties finishing a program that should have this output if we write 3 for example:
1
12
123
12
1
Or if we write 5 for example it should print out this:
1
12
123
1234
12345
1234
123
12
1

And I can do this program with for loop, but I have to use recursion, and here is what I have done so far:

public class Aufgabe3 {                          

    private static void printSequenz(int n) {                    
if(n<1){                  
    return;               
   }                 
        printMany(n);                 
        printSequenz(n-1);                  
    }                 


    private static void printMany(int n){                    
        for(int i=1;i<=n;i++){               
            System.out.print(i);                 
        }                  
        System.out.println();              
    }                 

    public static void main(String[] args) {              
printSequenz(5);              
    }              

}               

I would be really happy if someone would help me :).

Answer 1

You need to implement two recursive functions:

void printLoToHi(int n)
{
    if (n < 1)
        return;
    printLoToHi(n-1);
    printMany(n);
}

void printHiToLo(int n)
{
    if (n < 1)
        return;
    printMany(n);
    printHiToLo(n-1);
}

Then, you need to call them sequentially:

printSequenz(int n)
{
    printLoToHi(n);
    printHiToLo(n-1); // -1 in order to avoid printing the highest twice
}

Or in a more "symmetrical manner":

printSequenz(int n)
{
    printLoToHi(n-1); // print lowest to second highest
    printMany(n);     // print the highest
    printHiToLo(n-1); // print second highest to lowest
}

Answer 2

You could do it like this:

private static void printSequenz(int n) {


    printSequenz(1,n, true);

}

private static void printSequenz(int current, int total, boolean goingUp) {
    if(!goingUp && current<1){
        return;
    }

    printMany(current);
    if(current+1>total){
        goingUp=false;

    }
    if(goingUp){
        printSequenz(current+1,total,goingUp);
    } else {
        printSequenz(current-1,total,goingUp);
    }


}


private static void printMany(int n) {
    for (int i = 1; i <= n; i++) {
        System.out.print(i);
    }
    System.out.println();
}

public static void main(String[] args) {
    printSequenz(5);
}

Answer 3

public class Test { 
    public static void main(String args[]) {
        int seq = 6;
        for(int i=1; i<=seq; i++) {
            System.out.println("");
            int low =seq-(seq-i);
            printIncreasing(i,low);         
        }
        for(int i=seq-1; i>=1; i--) {
            System.out.println("");
            int low = seq-i;
            printDecreasing(i,low);         
        }
    }

    public static void printIncreasing(int high, int low) {
        for(int i = 1; i<=high;i++ ) {
            System.out.print(i);
        }       
    }

    public static void printDecreasing(int high, int low) {
        for(int i = 1; i<=high;i++ ) {
            System.out.print(i);
        }       
    }
}