【leetcode】1257. Smallest Common Region

题目如下：

You are given some lists of regions where the first region of each list includes all other regions in that list.

Naturally, if a region X contains another region Y then X is bigger than Y.

Given two regions region1, region2, find out the smallest region that contains both of them.

If you are given regions r1, r2 and r3 such that r1 includes r3, it is guaranteed there is no r2 such that r2 includes r3.

It's guaranteed the smallest region exists.

Example 1:

Input:
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"
Output: "North America"

Constraints:

• 2 <= regions.length <= 10^4
• region1 != region2
• All strings consist of English letters and spaces with at most 20 letters.

解题思路：首先递归找出region1所属的regions链，并保持结果；然后再递归查找region2所属的regions链，找到第一个region在region1所属的regions链即可。

代码如下：

class Solution(object):
    def findSmallestRegion(self, regions, region1, region2):
        """
        :type regions: List[List[str]]
        :type region1: str
        :type region2: str
        :rtype: str
        """
        dic = {}
        for region in regions:
            parent = region[0]
            for i in range(1,len(region)):
                dic[region[i]] = parent

        dic_region1 = {}
        while region1 in dic:
            dic_region1[region1] = 1
            region1 = dic[region1]

        while region2 in dic:
            if region2 in dic_region1:
                return region2
            region2 = dic[region2]
        return regions[0][0]