week5 51. N-Queens

public class Solution {
    public List<List<String>> solveNQueens(int n) {
        List<Deque<Integer>> result = this.nQueens(n);
       
        List<List<String>> resultAsStr = new ArrayList<List<String>>();
        
        for (Deque<Integer> variation: result){
            
            String[] board = new String[n];
            Integer[] temp = variation.toArray(new Integer[0]);
            
            for (int row=0;row<variation.size();++row){
                char[] currRow = new char[n];
                Arrays.fill(currRow, '.');
                Integer col = temp[row];
                currRow[col] = 'Q';
                board[row] = new String(currRow);                
            }
            resultAsStr.add(new ArrayList<String>(Arrays.asList(board)));
        }


        return resultAsStr;

    }

    private List<Deque<Integer>> nQueens(int n){
        
        List<Deque<Integer>> result = new ArrayList<Deque<Integer>>();
        Deque<Integer> colPlacement = new ArrayDeque<Integer>();
        this.nQueens(n, 0, colPlacement, result);
        return result;
    }

    private void nQueens(int n, int row, Deque<Integer> colPlacement,       List<Deque<Integer>> result){
        if (n==row){
            result.add(new ArrayDeque<Integer>(colPlacement));
        } else {

            for (int col=0;col<n;++col){
                colPlacement.add(col);
                if (this.isValid(colPlacement)){
                    this.nQueens(n, row+1, colPlacement, result);
                }
                colPlacement.pollLast();
            }
        }
    }

    private boolean isValid(Deque<Integer> colPlacement){
        int row_id = colPlacement.size() -1;
        Integer[] colArray = colPlacement.toArray(new Integer[0]);
        for (int i=0;i<row_id;++i){
            int diff = Math.abs(colArray[i] - colArray[row_id]);
            if (diff == 0 || diff == row_id -i){
                return false;
            }
        }
        return true;
    }

}