leetcode - N-Queens II

Follow up for N-Queens problem.

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

class Solution {
private:
    int res;
public:
    int totalNQueens(int n) {
        std::vector<int> state(n, -1);
        res = 0;
        dfs(state, 0);
        return res;
    }
    void dfs(std::vector<int> &state, int row)
    {
        int n = state.size();
        if(row == n)
        {
            res++;
            return;
        }
        for(int col = 0; col < n; col++)
            if(isValid(state, row, col))
            {
                state[row] = col;
                dfs(state, row+1);
                state[row] = -1;;
            }
    }
    bool isValid(std::vector<int> &state, int row, int col)
    {
        for(int i = 0; i < row; i++)
            if(state[i] == col || abs(row - i) == abs(col - state[i]))
                return false;
        return true;
    }
 
};