Not able to find possible solutions for N-Queens puzzle

Question

I am trying to find all possible solutions for N-Queens puzzle. I have to print single queen on each row, and no two queens should be adjacent to each other like no one or more queens diagonally, no one or more queens in a same row, no one or more queens in a same column.

I have written the algorithm and think that most part of it is correct, but the solutions are not getting printed. I don't understand why. I have spent a lot of time on this.

Any help will be appreciated. Following is my code:

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();

    }
}

Answer 1

Your code looks fine. It is only missing a key check in the validSol method. Change the validSold method to the following and your code should work fine.

public boolean validSol(int r, int n){
        if(r== n-1) {
            return true;
        }
        else {
            return false;
        }
    }

Let me know if this works for you.