232.Implement Queue using Stacks

tags: `Easy`,`Stack`,`Queue`

題目描述

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.

範例

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.

解答

Python
























class MyQueue:

    def __init__(self):
        self.input_stk = []
        self.output_stk = []

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

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

    def peek(self) -> int:
        if not self.output_stk:
            while self.input_stk:
                front = self.input_stk.pop()
                self.output_stk.append(front)

        peek = self.output_stk[-1]
        return peek

    def empty(self) -> bool:
        return not self.input_stk and not self.output_stk

KobeFri, Dec 16, 2022

C#


























public class MyQueue {
    Stack<int> target = new Stack<int>();
    Stack<int> temp = new Stack<int>();

    public void Push(int x) {
        while(target.Any()){
            temp.Push(target.Pop());
        }
        target.Push(x);
        while(temp.Any()){
            target.Push(temp.Pop());
        }
    }
    
    public int Pop() {
        return target.Pop();
    }
    
    public int Peek() {
        return target.Peek();
    }
    
    public bool Empty() {
        return !target.Any();
    }
}

JimFri, Dec 16, 2022

Javascript

































class MyQueue {
  constructor() {
    this.stack = [];
  }

  push(x) {
    this.stack.push(x);
  }

  // use two stacks to implement queue
  pop() {
    const temp = [];
    while (this.stack.length > 1) {
      temp.push(this.stack.pop());
    }

    const result = this.stack.pop();

    while (temp.length > 0) {
      this.stack.push(temp.pop());
    }

    return result;
  }

  peek() {
    return this.stack[0];
  }

  empty() {
    return this.stack.length === 0;
  }
}

Marsgoat Dec 16, 2022

Java



























class MyQueue {

    Stack<Integer> input = new Stack<Integer>();
    Stack<Integer> output = new Stack<Integer>();
    
    public void push(int x) {
        input.push(x);
    }
    
    public int pop() {
        peek();
        return output.pop();
    }
    
    public int peek() {
        if(output.empty()) {
            while(!input.empty()) {
                output.push(input.pop());
            }
        }
        return output.peek();
    }
    
    public boolean empty() {
        return input.empty() && output.empty();
    }
}