N-Queens

public class Solution {
    public List<String[]> solveNQueens(int n) {
        if(n <= 0)  return rst;
        char[][] board = new char [n][n];
        for(int i = 0; i < n; i++)
        {
            for(int j = 0; j < n; j++)
            {
                board[i][j] = ‘.‘;
            }
        }
        placeQueens(board, 0, n);
        return rst;
    }
    private List<String[]> rst = new ArrayList<String[]>();
    public void placeQueens(char[][] board, int row, int n)
    {
        //success
        if(row == n)
        {
            String[] strings = new String[n];
            for(int i = 0; i < n ; i++)
            {
                strings[i] = new String(board[i]);
            }
            rst.add(strings);
            return;
        }
        for(int j = 0; j < n; j++)
        {
            if(canPut(board, row, j, n))
            {
                board[row][j] = ‘Q‘;
                placeQueens(board, row + 1, n);
                board[row][j] = ‘.‘;
            }
        }
    }
    public boolean canPut(char [][]board, int x, int y, int n)
    {
        if(x == 0)  return true;
        for(int row = 0; row <= x - 1; row++)
        {
            for(int col = 0; col < n; col++)
            {
                char c = board[row][col];
                if(c == ‘.‘) continue;
                else
                {
                    if(y == col) return false;
                    if(col + row == y + x) return false;
                    if(col - row == y - x) return false;
                }
            }
        }
        return true;
    }
}