[Leetcode] N-Queens II

Follow up for N-Queens problem.

Now, instead outputting board configurations, return the total number of distinct solutions.

跟上一题一样，改下结果的显示方式。

class Solution {
public:
    bool isValid(vector<string> &board, int x, int y) {
        for (int i = 0; i < x; ++i) {
            if (board[i][y] == 'Q') return false;
        }
        for (int i = 0; i < board.size(); ++i) {
            for (int j = 0; j < board.size(); ++j) {
                if (i != x && j != y && i-j == x-y && board[i][j] == 'Q')
                    return false;
                if (i != x && j != y && i+j == x+y && board[i][j] == 'Q')
                    return false;
            }
        }
        return true;
    }
    
    void solveHelper(int &res, vector<string> &board, int idx) {
        if (idx == board.size()) {
            ++res;
            return;
        }
        for (int i = 0; i < board.size(); ++i) {
            board[idx][i] = 'Q';
            if (isValid(board, idx, i)) {
                solveHelper(res, board, idx + 1);
            }
            board[idx][i] = '.';
        }
    }
    
    int totalNQueens(int n) { 
        int res = 0;
        vector<string> board;
        string row;
        for (int i = 0; i < n; ++i) {
            row.push_back('.');
        }
        for (int i = 0; i < n; ++i) {
            board.push_back(row);
        }
        solveHelper(res, board, 0);
        return res;
    }
};