225. Implement Stack using Queues

Easy

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Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push, top, pop, and empty).

Implement the MyStack class:

• void push(int x) Pushes element x to the top of the stack.

• int pop() Removes the element on the top of the stack and returns it.

• int top() Returns the element on the top of the stack.

• boolean empty() Returns true if the stack is empty, false otherwise.

Notes:

• You must use only standard operations of a queue, which means that only push to back, peek/pop from front, size and is empty operations are valid.

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

Example 1:

Input
["MyStack", "push", "push", "top", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 2, 2, false]

Explanation
MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // return 2
myStack.pop(); // return 2
myStack.empty(); // return False

Constraints:

• 1 <= x <= 9

• At most 100 calls will be made to push, pop, top, and empty.

• All the calls to pop and top are valid.

Follow-up: Can you implement the stack using only one queue?

Solution 1 :

class MyStack:

    def __init__(self):
        self.q = []
        self.temp = []
        

    def push(self, x: int) -> None:
        self.q.append(x)
        
    def pop(self) -> int:
        for index in range(len(self.q)-1):
            self.temp.append(self.q[index])
        value = self.q[-1]
        self.q = self.temp
        self.temp = []
        return value
        

    def top(self) -> int:
        return self.q[-1]
        

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


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

Solution 2 :

class MyStack:

    def __init__(self):
        self.q = deque()
        

    def push(self, x: int) -> None:
        self.q.append(x)
        for _ in range(len(self.q)-1):
            self.q.append(self.q.popleft())
        
    def pop(self) -> int:
        return self.q.popleft()
        

    def top(self) -> int:
        return self.q[0]
        

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


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