leetcode52. N-Queens II

Follow up for N-Queens problem.

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

class Solution {
public:
    int totalNQueens(int n) {
        int res = 0;
        vector<string> nQueens(n, string(n, '.'));
        dfs(res, nQueens, 0, n);

        return res;
    }
private:
    void dfs(int& res, vector<string>& nQueens, int row, int n) {
        if (row == n) {
            res++;
            return;
        }
        for (int col = 0; col < n; col++) {
            if(isValid(nQueens,row,col,n)){
                nQueens[row][col] = 'Q';
                dfs(res,nQueens,row+1,n);
                nQueens[row][col] = '.';
            }
        }
    }

    bool isValid(vector<string>& nQueens,int row,int col,int n){
        for(int i=0;i<row;i++){
            if(nQueens[i][col] == 'Q')
                return false;
        }

        for(int i=row-1,j=col-1;i>=0 && j>=0;--i,--j){
            if(nQueens[i][j] == 'Q')
                return false;
        }

        for(int i = row-1,j=col+1;i>=0 && j < n;--i,++j){
            if(nQueens[i][j] == 'Q')
                return false;
        }
        return true;
    }
};