递归方法如何开始打印整形

Question

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I want to know how recursive method work to prints a large X composed of smaller X's with a given “width”, input number, which is guaranteed to be odd.

the “width” is length (number) of X's along one line large X.

Example for an X of width input number =3 the method will print this shape!

 X X

  X

 X X

I try to solve this problem but I couldn't can anyone here help me .. in java code ,

This is my code he works good but prints wrong when numberinput=7 or 5

 public static String shape(String i,int  numberinput) {   
 //error check, not working for even numbers
 if(numberinput%2 == 0)
    return null;

 //terminating condition, stop recursion when this occurs.
 if(numberinput == 1)
    return "X";

else

return "X"+" "+i+"\n" +" "+shape(" "+i,numberinput-2)+" "+"\n"+i+" "+"X";
}

he prints this when numberinput=5

       X X

    X  X

    X 

    X X 

    X X

Answer 1

A valid recursive method should have two parts.

1. Recursive call (call itself to do part of work)
2. Terminating condition (A condition to stop the recursion)

You have a recursive call, but not a termination condition. Hence your recursion won't stop until it fills up the entire stack and cause an exception. Hence you should include a terminating condition in your recursive method.

A sample implementation might look like this.

public static String shap(String i, int numberinput) {
    //error check, not working for even numbers
    if(numberinput%2 == 0)
        return null;

    //terminating condition, stop recursion when this occurs.
    if(numberinput == 1)
        return "X";

    //recursion, call recursive until terminating condition occurs.
    return "X" + i + shap(i, numberinput-2) + i + "X";
}

Answer 2

I have written a java code, in which recursion is only for the levels, to generate the string i have used a loop :

import java.util.*;
import java.lang.*;
import java.io.*;

public class Main
{
    public static void main (String[] args) throws java.lang.Exception
    {
        List<String> ans = new ArrayList<String>();
        shap(7, 1, ans);
        //System.out.println(ans);
        for(int i = 0;i < ans.size();i++){
            System.out.println(ans.get(i));
        }
    }

    public static void shap(int numberinput, int currentLevel, List<String> ans) {
        if(currentLevel == numberinput+1) return;
        String val = "";
        for(int i = 1;i <= numberinput;i++){
            if(i == currentLevel || i == (numberinput+1-currentLevel)) val += "X";
            else val += " ";
        }
        ans.add(val);
        shap(numberinput, currentLevel+1, ans);//Recursion step for the levels
    }
}

Link to solution on Ideone : http://ideone.com/RioL9g