Stack & Queue

Posted 2021-02-27 ybxw

 

232. Implement Queue using Stacks

FILO to FIFO

MyQueue queue = new MyQueue();

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

class MyQueue:

    def __init__(self):
        self.s1 = []
        self.s2 = []

    def push(self, x: int) -> None:
        while self.s1:
            self.s2.append(self.s1.pop())
        self.s1.append(x)
        while self.s2:
            self.s1.append(self.s2.pop())

    def pop(self) -> int:
        return self.s1.pop()

    def peek(self) -> int:
        return self.s1[-1]
                    
    def empty(self) -> bool:
        return not 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()

Queue push O(n) pop O(1) peek O(1)

class MyQueue:

    def __init__(self):
        self.s1 = []
        self.s2 = []

    def push(self, x: int) -> None:
        self.s1.append(x)

    def pop(self) -> int:
        self.peek()
        return self.s2.pop()

    def peek(self) -> int:
        if not self.s2:
            while self.s1:
                self.s2.append(self.s1.pop())
        return self.s2[-1]

    def empty(self) -> bool:
        return not self.s1

Queue push O(1) pop O(1) peek O(n)

 

225. Implement Stack using Queues

FIFO to FILO

MyStack stack = new MyStack();

stack.push(1);
stack.push(2);  
stack.top();   // returns 2
stack.pop();   // returns 2
stack.empty(); // returns false

 

class Stack:

    def __init__(self):
        self._queue = collections.deque()

    def push(self, x):
        q = self._queue
        q.append(x)
        for _ in range(len(q) - 1):
            q.append(q.popleft())
        
    def pop(self):
        return self._queue.popleft()

    def top(self):
        return self._queue[0]
    
    def empty(self):
        return not len(self._queue)

Stack