leetcode 232. Implement Queue using Stacks

Posted 2020-11-17 将者，智、信、仁、勇、严也。

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

Example:

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);  
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false

Notes:

• You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is empty operations are valid.
• Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

 

class MyQueue:

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.stack1 = []
        self.stack2 = []
        

    def push(self, x):
        """
        Push element x to the back of queue.
        :type x: int
        :rtype: void
        """
        self.stack1.append(x)
    
    def dump_data(self):
        if not self.stack2:
            while self.stack1:
                self.stack2.append(self.stack1.pop())

    def pop(self):
        """
        Removes the element from in front of queue and returns that element.
        :rtype: int
        """
        self.dump_data()
        return self.stack2.pop()
        

    def peek(self):
        """
        Get the front element.
        :rtype: int
        """
        self.dump_data()
        return self.stack2[-1]
        

    def empty(self):
        """
        Returns whether the queue is empty.
        :rtype: bool
        """
        return not self.stack1 and not self.stack2


# 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()