Leetcode 51 Solution

class Solution {
    vector<vector<string>> m_res;
public:
    void solveNQueens(vector<int>& board, int n, int layer)
    {
        if (board.size() == layer)
        {
            vector<string> res;

            for (int i = 0; i < board.size(); i++)
            {
                string str = string(board[i], '.') + "Q" + string(n - board[i] - 1, '.');
                res.push_back(str);
            }

            m_res.push_back(res);
            return;
        }

        for (int i = 0; i < n; i++)
        {
            board[layer] = i;

            bool valid = true;
            for (int j = 0; j < layer; j++)
            {
                if (board[j] == i || abs(board[j] - i) == abs(j - layer))
                {
                    valid = false;
                    break;
                }
            }

            if (valid)
                solveNQueens(board, n, layer + 1);
        }
    }

    vector<vector<string>> solveNQueens(int n) {
        vector<int> board(n, -1);
        solveNQueens(board, n, 0);
        return m_res;
    }
};