How would you implement a stack using two queues?

Use two queues q1 and q2. For push: add element to q2, then move all elements from q1 to q2, then swap q1 and q2. For pop: simply dequeue from q1. This makes push O(n) and pop O(1). Alternative approach: make push O(1) and pop O(n) by adding to q1 directly, and for pop, move all except last element to q2, dequeue last, then swap.