Implement Queue using Stacks(Stack,Design,Queue)

https://leetcode.com/problems/implement-queue-using-stacks/description/

Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push, peek, pop, and empty).

Implement the MyQueue class:

• void push(int x) Pushes element x to the back of the queue.
• int pop() Removes the element from the front of the queue and returns it.
• int peek() Returns the element at the front of the queue.
• boolean empty() Returns true if the queue is empty, false otherwise.

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, the stack may not be supported natively. You may simulate a stack using a list or deque (double-ended queue) as long as you use only a stack's standard operations.

 

Example 1:

Input
["MyQueue", "push", "push", "peek", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 1, 1, false]

Explanation
MyQueue myQueue = new MyQueue();
myQueue.push(1); // queue is: [1]
myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
myQueue.peek(); // return 1
myQueue.pop(); // return 1, queue is [2]
myQueue.empty(); // return false

 

Constraints:

• 1 <= x <= 9
• At most 100 calls will be made to push, pop, peek, and empty.
• All the calls to pop and peek are valid.

 

Follow-up: Can you implement the queue such that each operation is amortized O(1) time complexity? In other words, performing n operations will take overall O(n) time even if one of those operations may take longer.

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뭐.. 걍 어찌어찌 했는데 이게 잘 한건지는 모르겄다

 

1. Python

class MyQueue:

    def __init__(self):
        self.s1 = [] # main memory
        self.s2 = [] # helper
    def push(self, x: int) -> None:
        self.s1.append(x)

    def pop(self) -> int:
        while self.s1:
            self.s2.append(self.s1.pop())
        temp = self.s2.pop()
        while self.s2:
            self.s1.append(self.s2.pop())
        return temp
    def peek(self) -> int:
        while self.s1:
            self.s2.append(self.s1.pop())
        temp = self.s2[-1]
        while self.s2:
            self.s1.append(self.s2.pop())
        return temp     
    def empty(self) -> bool:
        return self.s1 == []


# 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()

 

2. C/C++로도 함 해보기!