#include <iostream> #include <vector> using namespace std; int main() { vector<string> container, undo; //undo数组保存被回滚的字符串 string str; while (cin >> str) { if (str == "undo") { if (!container.empty()) //回滚操作只能发生在container非空的情况下 { undo.push_back(container.back()); con...