232. Implement Queue using Stacks(栈实现队列)

请你仅使用两个栈实现先入先出队列。队列应当支持一般队列支持的所有操作（push、pop、peek、empty）：

实现 MyQueue 类：

void push(int x) 将元素 x 推到队列的末尾
int pop() 从队列的开头移除并返回元素
int peek() 返回队列开头的元素
boolean empty() 如果队列为空，返回 true ；否则，返回 false
 

说明：

你只能使用标准的栈操作 —— 也就是只有 push to top, peek/pop from top, size, 和 is empty 操作是合法的。
你所使用的语言也许不支持栈。你可以使用 list 或者 deque（双端队列）来模拟一个栈，只要是标准的栈操作即可。

 

 

class MyQueue:

    def __init__(self):
        self.A, self.B = [], []

    def push(self, x: int) -> None:
        self.A.append(x)

    def pop(self) -> int:
        peek = self.peek()
        self.B.pop()
        return peek

    def peek(self) -> int:
        if self.B: return self.B[-1]
        if not self.A: return -1
        # 将栈 A 的元素依次移动至栈 B
        while self.A:
            self.B.append(self.A.pop())
        return self.B[-1]

    def empty(self) -> bool:
        return not self.A and not self.B

 

 

class MyQueue:

    def __init__(self):
        self.stack1 = []
        self.stack2 = []

    def push(self, x: int) -> None:
        #push
        self.stack1.append(x)

    def pop(self) -> int:
        while len(self.stack1) != 0:
            self.stack2.append(self.stack1.pop())
        top = self.stack2.pop()

        while len(self.stack2) != 0:
            self.stack1.append(self.stack2.pop())
        return top
    def peek(self) -> int:
        while len(self.stack1) != 0:
            self.stack2.append(self.stack1.pop())
        top = self.stack2[-1]
        while len(self.stack2) != 0:
            self.stack1.append(self.stack2.pop())
        return top

    def empty(self) -> bool:
        return len(self.stack1) == 0


# Your MyQueue object will be instantiated and called as such:
# obj = MyQueue()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.peek()
# param_4 = obj.empty()

 

 

 

 

 

 

 

 

 

 

class MyQueue {
private:
    stack<int> s1;
    stack<int> s2;
public:
    /** Initialize your data structure here. */
    MyQueue() {

    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        s2.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        int v = peek();
        s1.pop();
        return v;
    }
    
    /** Get the front element. */
    int peek() {
        if(s1.empty()) {
            while(!s2.empty()) {
                s1.push(s2.top());
                s2.pop();
            }
        }
        return s1.top();
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return s1.empty() && s2.empty();
    }
};

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue* obj = new MyQueue();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->peek();
 * bool param_4 = obj->empty();
 */