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use std::collections::VecDeque;
pub struct Stack<T>(VecDeque<T>, VecDeque<T>);
// use two queues for a stack.
// enqueue to the first stack, dequeue off the second.
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use std::collections::VecDeque;
pub struct Stack<T>(VecDeque<T>, VecDeque<T>);
// use two queues for a stack.
// enqueue to the first stack, dequeue off the second.