Leetcode: N-Queens

he 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.."]
]

Understand the problem:
It is a classic N-queue problem. The eight queens puzzle is the problem of placing eight chess queens on an 8×8 chessboard so that no two queens threaten each other. Thus, a solution requires that no two queens share the same row, column, or diagonal. The eight queens puzzle is an example of the more general n-queens problem of placing n queens on an n×n chessboard, where solutions exist for all natural numbers n with the exception of n=2 or n=3. 

Solution:
This is again a classic permutation and combination problem. So we can still apply the same recursive approach. 

Code (Java):

public class Solution {
    public List<String[]> solveNQueens(int n) {
        List<String[]> result = new ArrayList<String[]>();
        
        if (n == 0 || n == 2 || n == 3) {
            return result;
        }
        
        solveNqueensHelper(n, 0, new int[n], result);
        
        return result;
    }
    
    private void solveNqueensHelper(int n, int row, int[] cols, List<String[]> result) {
        if (row == n) {
            String[] solution = new String[n];
            for (int i = 0; i < n; i++) {
                StringBuilder temp = new StringBuilder();
                for (int j = 0; j < n; j++) {
                    if (cols[i] == j) {
                        temp.append('Q');
                    } else {
                        temp.append('.');
                    }
                }
                solution[i] = temp.toString();
            }
            result.add(solution);
            return;
        }
        
        for (int j = 0; j < n; j++) {
            if (isValid(j, row, cols) == true) {
                cols[row] = j;
                solveNqueensHelper(n, row + 1, cols, result);
            }
        }
    }
    
    private boolean isValid(int col, int row, int[] cols) {
        for (int i = 0; i < row; i++) {
            if (col == cols[i] || row - i == Math.abs(col - cols[i])) {
                return false;
            }
        }
        return true;
    }
}