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class FIFO { private: LIFO l1; LIFO l2; public: void enqueue(int n) { l1.push(n); } int isempty() { return l1.isempty() && l2.isempty(); } int dequeue() { if (l2.empty()) { while (!l1.empty()) { l2.push(l1.pop()); } } return l2.pop(); } };