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// @Title: 用两个栈实现队列 (用两个栈实现队列 LCOF)
// @Author: 15816537946@163.com
// @Date: 2022-01-31 11:28:26
// @Runtime: 60 ms
// @Memory: 4.7 MB
// 第9题的第二种题解, 借鉴网上的答案
#[derive(Default)]
struct CQueue {
    p: Vec<i32>,
    q: Vec<i32>,
}

/**
 * `&self` means the method takes an immutable reference.
 * If you need a mutable reference, change it to `&mut self` instead.
 */
impl CQueue {
    fn new() -> Self {
        Default::default()
    }

    fn append_tail(&mut self, value: i32) {
        self.p.push(value);
    }

    fn delete_head(&mut self) -> i32 {
        match self.q.pop() {
            Some(v) => v,
            None => {
                self.p.reverse();
                self.q.append(&mut self.p);
                self.q.pop().unwrap_or(-1)
            }
        }
    }
}

/*
 * Your CQueue object will be instantiated and called as such:
 * let obj = CQueue::new();
 * obj.append_tail(value);
 * let ret_2: i32 = obj.delete_head();
 */