1436. Destination City - Explanation

class Solution:
    def destCity(self, paths: List[List[str]]) -> str:
        for i in range(len(paths)):
            flag = True
            for j in range(len(paths)):
                if paths[i][1] == paths[j][0]:
                    flag = False
                    break
            if flag:
                return paths[i][1]
        return ""

class Solution:
    def destCity(self, paths: List[List[str]]) -> str:
        s = set()
        for p in paths:
            s.add(p[0])

        for p in paths:
            if p[1] not in s:
                return p[1]

class Solution:
    def destCity(self, paths: List[List[str]]) -> str:
        mp = {p[0]: p[1] for p in paths}

        start = paths[0][0]
        while start in mp:
            start = mp[start]
        return start