LeetCode 1257. 最小公共区域（最小公共祖先）

示例 1：
输入：
regions = [["Earth","North America","South America"],
["North America","United States","Canada"],
["United States","New York","Boston"],
["Canada","Ontario","Quebec"],
["South America","Brazil"]],
region1 = "Quebec",
region2 = "New York"
输出："North America"
 
提示：
2 <= regions.length <= 10^4
region1 != region2
所有字符串只包含英文字母和空格，且最多只有 20 个字母。

class Solution {
public:
    string findSmallestRegion(vector<vector<string>>& regions, string region1, string region2) {
    	unordered_map<string,string> father;
    	for(auto& r : regions)
    		for(int i = 1; i < r.size(); ++i)
    			father[r[i]] = r[0];
    	unordered_set<string> f1;
    	while(region1 != "")
    	{
    		f1.insert(region1);
    		region1 = father[region1];
    	}
    	while(!f1.count(region2))
    		region2 = father[region2];
    	return region2;
    }
};