[LeetCode] N-Queens II

Follow up for N-Queens problem.

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

class Solution {
public:
    bool isOK(vector<int> &vec, int index)
    {
        int size = vec.size();
        if(0 == size) return true;
        for(int i=0;i<size;i++)
        {
            if(vec[i] == index) return false;
            if(abs(index - vec[i])==size-i) return false;
        }
        return true;
    }

    void solve(int &num, vector<int> &vec, int n)
    {
        if(vec.size() == n)
        {
            num++;
            return;
        }
        for(int i=0; i<n;i++)
        {
            if(!isOK(vec,i)) continue;
            vec.push_back(i);
            solve(num, vec, n);
            vec.pop_back();
        }
    }
    int totalNQueens(int n) 
    {
        int num = 0;
        vector<int> vec;
        solve(num, vec, n);
        return num;
    }
};