LeetCode – Refresh – N-Queens

Brute Force:

class Solution {
public:
    bool notExist(vector<int> rec, int level, int current) {
        for (int i = 0; i < level; i++) {
            if (rec[i] == current) return false;
            if (fabs(rec[i] - current) == fabs(level - i)) return false;
        }
        return true;
    }
    void genQueen(vector<int> rec, vector<string> &result) {
        for (int i = 0; i < rec.size(); i++) {
            ostringstream oss;
            for (int j = 0; j < rec.size(); j++) {
                if (rec[i] == j) oss << 'Q';
                else oss << '.';
            }
            result.push_back(oss.str());
        }
    }
    void getQueen(vector<vector<string> > &result, vector<int> rec, int n, int level) {
        if (level == n) {
            result.push_back(vector<string>());
            genQueen(rec, result.back());
            return;
        }
        for (int i = 0; i < n; i++) {
            if (notExist(rec, level, i)) {
                rec[level] = i;
                getQueen(result, rec, n, level+1);
            }
        }
    }
    vector<vector<string> > solveNQueens(int n) {
        vector<vector<string> > result;
        getQueen(result, vector<int> (n,0), n, 0);
        return result;
    }
};