[C++] Find the CityB not in CityA
Intuition\n Describe your first thoughts on how to solve this problem. \n- The destination city is the city without any path outgoing to another city\n- So the 
# Intuition\n<!-- Describe your first thoughts on how to solve this problem. -->\n- The destination city is the city without any path outgoing to another city\n- So the destination city must not belong to `CityAi`\n\n# Approach\n<!-- Describe your approach to solving the problem. -->\n- Put all `cityAi` into a set\n- Find the `cityBi` that is not in the set\n\n# Complexity\n- Time complexity: $$O(n)$$\n<!-- Add your time complexity here, e.g. $$O(n)$$ -->\n\n- Space complexity: $$O(n)$$\n<!-- Add your space complexity here, e.g. $$O(n)$$ -->\n\n# Code\n```\nclass Solution {\npublic:\n    // time/space: O(n)/O(n)\n    string destCity(vector<vector<string>>& paths) {\n        unordered_set<string> cityA;\n        for (auto& path : paths) cityA.insert(path[0]);\n        for (auto& path : paths) {\n            if (cityA.count(path[1]) == 0) return path[1];\n        }\n        return "";\n    }\n};\n```
['C++']