[daily question] chessboard question

Instruction
On a chessboard of a given shape （ The shape may be irregular ） There are chess pieces on it , There is no difference in chess . It is required that any two pieces should not be placed in the same line or column in the chessboard , Please program to solve for the given shape and size of the chessboard , put k All the feasible placement plans of a piece C.
Input
The input contains multiple sets of test data .
The first row of each set of data is two positive integers ,n k, Separated by a space , Indicates that there will be a n*n The matrix of describes the chessboard , And the number of pieces placed . n <= 8 , k <= n
When it comes to -1 -1 Is the end of the input .
And then n Line describes the shape of the chessboard ： Each row has n Characters , among # Represents the board area , . Indicates a blank area （ Data is guaranteed to be free of redundant blank rows or columns ）.
Output
For each set of data , Give a line of output , Number of output placement schemes C （ Data assurance C<2^31）.
Sample

Answer

import java.util.Scanner;
import javax.sound.midi.SoundbankResource;

/** * @author liangyuanshao * @date 2022/4/21 - 17:46 */
public class Main {
    
  static Boolean[][] arr;
  static int n,k;
  static int count=0;
  public static void main(String[] args) {
    
    Scanner s=new Scanner(System.in);
    while (true){
    
       n=s.nextInt(); k=s.nextInt();
      if(n!=-1){
    
        arr=new Boolean[n][n];
        for(int i=0;i<n;i++){
    
          String temp=s.next();
          for(int j=0;j<temp.length();j++){
    
            arr[i][j]=temp.charAt(j)=='#';
          }
        }
        count=0;
        dfs(0,new boolean[n],k);
        System.out.println(count);
      }else {
    
        break;
      }
    }
    s.close();
  }
  public static void dfs(int row,boolean[] col,int num){
    
    if(num==0){
    
      count++;
      return;
    }
    if(row==n){
    
      return ;
    }
    dfs(row+1,col,num);
    for(int j=0;j<n;j++){
    
      if(arr[row][j]&&!col[j]){
    
        col[j]=true;
        dfs(row+1,col,num-1);
        col[j]=false;
      }
    }
  }
}

Experience
Java Whether a function passes a value or a reference , The result of modification in the function , Will it affect the caller
for instance , Why does the code with red box exist in the following line ？

1. The basic variable is the passed reference , Do not change the caller itself , image int,boolean,float etc.
2. An object is a passed reference , Both the function and the caller are the same object of the operation . Because arrays are objects , So the top should be right col After array modification , Then modify it back
3. however String More special , Although it's an object , But what is still passed is a reference , because String Yes, it is final modification , It is not modifiable by default . When a function is passed , In fact, it opens another space and transmits the same String object .