LeetCode - N-Queens II

class Solution {
public:
    int totalNQueens(int n) {
        int res = 0;
        vector<int> visited(n, -1);
        helper(n, 0, visited, res);
        return res;
    }
    void helper(int n, int row, vector<int>& visited, int& res){
        if(row==n) res++;
        else{
            for(int i=0;i<n;i++){
                if(isvalid(visited, row, i)){
                    visited[row] = i;
                    helper(n, row+1, visited, res);
                    visited[row] = -1;
                }
            }
        }
    }
    bool isvalid(vector<int>& visited, int row, int col){
        if(visited[row]!=-1) return false;
        for(int i=0;i<row;i++){
            if(visited[i]==col || abs(visited[i]-col)==abs(i-row)) return false;
        }
        return true;
    }
};