NQueenBruteJava

class Solution {
    public List<List<String>> solveNQueens(int n) {
        char[][] board = new char[n][n];
        for(int i = 0; i < n; i++)
            for(int j = 0; j < n; j++)
                board[i][j] = '.';
        List<List<String>> res = new ArrayList<List<String>>();
        dfs(0, board, res);
        return res;
    }
    
    private boolean validate(char[][] board, int row, int col) {
        int duprow = row;
        int dupcol = col; 
        while(row >= 0 && col >= 0) {
            if(board[row][col] == 'Q') return false; 
            row--;
            col--; 
        }
        
        row = duprow; 
        col = dupcol; 
        while(col >= 0) {
            if(board[row][col] == 'Q') return false; 
            col--; 
        }
        
        row = duprow; 
        col = dupcol; 
        while(col >= 0 && row < board.length) {
            if(board[row][col] == 'Q') return false; 
            col--;
            row++; 
        }
        return true; 
    }
    
    private void dfs(int col, char[][] board, List<List<String>> res) {
        if(col == board.length) {
            res.add(construct(board));
            return;
        }
        
        for(int row = 0; row < board.length; row++) {
            if(validate(board, row, col)) {
                board[row][col] = 'Q';
                dfs(col+1, board, res);
                board[row][col] = '.';
            }
        }
    }
    
    
    
    private List<String> construct(char[][] board) {
        List<String> res = new LinkedList<String>();
        for(int i = 0; i < board.length; i++) {
            String s = new String(board[i]);
            res.add(s);
        }
        return res;
    }
}