Find the Root Cause Node from Parent-to-Children Logs

You are given logs describing a parent-to-children relationship that forms a tree (or a forest). Each record has the form: [node, [child1, child2, ...]], meaning directed edges node -> child for each listed child. Find the root node: the node with in-degree 0 (it never appears as any other node’s child). Input (stdin) The first line contains an integer n, the number of records. Then n lines follow, each formatted as: node k child1 child2 ... childk where k is the number of children. Output (stdout) Print the root node identifier. Constraints 1 <= n <= 2*10^5 Total number of nodes V <= 2*10^5 Node identifiers are whitespace-free strings Assume there is exactly one root (you may ignore invalid inputs) Sample Tests See the Chinese section. Example Input 3 A 2 B C B 1 D C 0 Output A