Leetcode: N-Queens

The n-queens puzzle is the problem of placing n queens on an n×n chessboard such that no two queens attack each other.

Given an integer n, return all distinct solutions to the n-queens puzzle.
Each solution contains a distinct board configuration of the n-queens' placement, where 'Q' and '.' both indicate a queen and an empty space respectively.
For example,
There exist two distinct solutions to the 4-queens puzzle:

[
 [".Q..",  // Solution 1
  "...Q",
  "Q...",
  "..Q."],

 ["..Q.",  // Solution 2
  "Q...",
  "...Q",
  ".Q.."]
]

Solution

public class Solution {
    public List<String[]> solveNQueens(int n) {
        boolean[][] board = new boolean[n][n];
        List<String[]> solution = new ArrayList<String[]>();
        solveNQueens(board, 0, solution);
        return solution;
    }
    
    public void solveNQueens(boolean[][]board, int row, List<String[]> solution)
    {
        if(row==board.length)
        {
            String[] oneSolution = new String[board.length];
            for(int i=0; i<board.length;i++)
            {
                StringBuilder sb = new StringBuilder();
                for(int j=0; j<board[0].length; j++)
                    if(board[i][j])
                        sb.append('Q');
                    else
                        sb.append('.');
                oneSolution[i] = sb.toString();
            } 
            solution.add(oneSolution);
            return;
        }
        for(int i=0; i<board[0].length; i++)
        {
            if(canAdd(board, row, i))
            {
                board[row][i]=true;
                solveNQueens(board, row+1, solution);
                board[row][i]=false;
            }
        }
    }
    
    private boolean canAdd(boolean[][] board, int row, int col)
    {
        for(int i=0; i<row; i++)
        {
            if(board[i][col])
                return false;
            if(col+i-row>=0 && board[i][col+i-row])
                return false;
            if(col+row-i<board[0].length && board[i][col+row-i])
                return false;
        }
        return true;
    }
}