232. Implement Queue using Stacks

# Easy

straight forward, stack1 is the main stack, stack2 is the help stack. In push() and peek(), pop all elements of stack1 to stack2, and then push back to stack1.

Python

class MyQueue:

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

    def push(self, x: int) -> None:
        """
        Push element x to the back of queue.
        """
        self.stack1.append(x)
        

    def pop(self) -> int:
        """
        Removes the element from in front of queue and returns that element.
        """
        while len(self.stack1) != 1:
            self.stack2.append(self.stack1.pop())
        tmp = self.stack1.pop()
        while len(self.stack2) != 0:
            self.stack1.append(self.stack2.pop())
        return tmp
        

    def peek(self) -> int:
        """
        Get the front element.
        """
        while len(self.stack1) != 1:
            self.stack2.append(self.stack1.pop())
        tmp = self.stack1.pop()
        self.stack1.append(tmp)
        while len(self.stack2) != 0:
            self.stack1.append(self.stack2.pop())
        return tmp       
        

    def empty(self) -> bool:
        """
        Returns whether the queue is empty.
        """
        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()