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  • [LC] 232. Implement Queue using Stacks

    Implement the following operations of a queue using stacks.

    • push(x) -- Push element x to the back of queue.
    • pop() -- Removes the element from in front of queue.
    • peek() -- Get the front element.
    • empty() -- Return whether the queue is empty.

    Example:

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

    Notes:

    • You must use only standard operations of a stack -- which means only push to toppeek/pop from topsize, and is empty operations are valid.
    • Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
    • You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).
    class MyQueue:
    
        def __init__(self):
            """
            Initialize your data structure here.
            """
            self.stack_in, self.stack_out = [], []
    
        def push(self, x: int) -> None:
            """
            Push element x to the back of queue.
            """
            self.stack_in.append(x)
    
        def pop(self) -> int:
            """
            Removes the element from in front of queue and returns that element.
            """
            self._transfer()
            return self.stack_out.pop()
    
        def peek(self) -> int:
            """
            Get the front element.
            """
            self._transfer()
            return self.stack_out[-1]
        
        def empty(self) -> bool:
            """
            Returns whether the queue is empty.
            """
            return not self.stack_in and not self.stack_out
            
        def _transfer(self):
            if not self.stack_out:
                while self.stack_in:
                    self.stack_out.append(self.stack_in.pop())
            
    
    
    # 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()
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  • 原文地址:https://www.cnblogs.com/xuanlu/p/11776120.html
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