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June 16, 2022 02:30 pm GMT

Implement Stack using Queues

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:

class MyStack:    def __init__(self):        self.stack = []    def push(self, x: int) -> None:        self.stack.append(x)    def pop(self) -> int:        if not self.empty():            return self.stack.pop()    def top(self) -> int:        if not self.empty():            return self.stack[-1]    def empty(self) -> bool:        return len(self.stack) == 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()

Original Link: https://dev.to/theabbie/implement-stack-using-queues-al3

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