问无法找到N-皇后谜题的可能解决方案

EN

public class NQueens {

    int[] x;

    public NQueens(int n) {
        x = new int[n];
    }    // CONSTRUCTOR

    public void printQueens(int[] x) {
        int N = x.length;
        for (int i = 0; i < N; i++) {
            for (int j = 0; j < N; j++) {
                if (x[i] == j) {
                    System.out.print("Q ");
                } else {
                    System.out.print("* ");
                }
            }
            System.out.println();
        }
        System.out.println();
    }

    //Promising method
    public boolean canPlaceQueen(int r, int c) {

        for (int i = 0; i < r; i++) {
            if (x[i] == c || (i - r) == (x[i] - c) ||(i - r) == (c - x[i]))
            {
                return false;
            }
        }
        return true;
    }
    //permute method
    public void placeNqueens(int r, int n) {

        for (int c = 0; c < n; c++) {
            if (canPlaceQueen(r, c)) {
                x[r] = c;
                if (validSol(r,n)) {
                    printQueens(x);
                } else {
                    placeNqueens(r + 1, n);
                }
            }
        }
    }

    public boolean validSol(int r, int n){
        if(r== n) {
            return true;
        }
        else {
            return false;
        }
    }

    public void callplaceNqueens() {
        placeNqueens(0, x.length);
    }

    public static void main(String args[]) {
        NQueens Q = new NQueens(8);
        Q.callplaceNqueens();

    }
}