232. Implement Queue using Stacks

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.

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.

class MyQueue {
  input: number[];
  output: number[];

  constructor() {
    this.input = [];
    this.output = [];
  }

  push(x: number): void {
    this.input.push(x);
  }

  pop(): number {
    this.peek();
    return this.output.pop();
  }

  peek(): number {
    if (this.output.length === 0) {
      while (this.input.length) {
        this.output.push(this.input.pop());
      }
    }

    return this.output[this.output.length - 1];
  }

  empty(): boolean {
    return this.output.length === 0 && this.input.length === 0;
  }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * var obj = new MyQueue()
 * obj.push(x)
 * var param_2 = obj.pop()
 * var param_3 = obj.peek()
 * var param_4 = obj.empty()
 */

class MyQueue:
    def __init__(self):
        self.input = []
        self.output = []

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

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

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

    def empty(self) -> bool:
        return not self.output and not self.input


# 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()

class MyQueue {
public:
    stack<int> input;
    stack<int> output;

    MyQueue() {}

    void push(int x) { input.push(x); }

    int pop() {
        int val = peek();
        output.pop();
        return val;
    }

    int peek() {
        if (output.empty()) {
            while (!input.empty()) {
                int val = input.top();
                input.pop();
                output.push(val);
            }
        }

        return output.top();
    }

    bool empty() { return input.empty() && output.empty(); }
};

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue* obj = new MyQueue();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->peek();
 * bool param_4 = obj->empty();
 */

class MyQueue {
    private Stack<Integer> input = new Stack<>();
    private Stack<Integer> output = new Stack<>();

    public MyQueue() {

    }

    public void push(int x) {
        input.push(x);
    }

    public int pop() {
        peek();
        return output.pop();
    }

    public int peek() {
        if (output.empty()) {
            while (!input.empty()) {
                int val = input.pop();
                output.push(val);
            }
        }

        return output.peek();
    }

    public boolean empty() {
        return input.empty() && output.empty();
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * boolean param_4 = obj.empty();
 */

type MyQueue struct {
    input []int
    output []int
}


func Constructor() MyQueue {
    return MyQueue{}
}


func (this *MyQueue) Push(x int)  {
    this.input = append(this.input, x)
}


func (this *MyQueue) Pop() int {
    val := this.Peek()
    this.output = this.output[:len(this.output) - 1]
    return val
}


func (this *MyQueue) Peek() int {
    if len(this.output) == 0 {
        for len(this.input) > 0 {
            this.output = append(this.output, this.input[len(this.input) - 1])
            this.input = this.input[:len(this.input) - 1]
        }
    }
    return this.output[len(this.output) - 1]
}


func (this *MyQueue) Empty() bool {
    return len(this.input) == 0 && len(this.output) == 0
}


/**
 * Your MyQueue object will be instantiated and called as such:
 * obj := Constructor();
 * obj.Push(x);
 * param_2 := obj.Pop();
 * param_3 := obj.Peek();
 * param_4 := obj.Empty();
 */

#[derive(Default)]
struct MyQueue {
    input: Vec<i32>,
    output: Vec<i32>,
}


/**
 * `&self` means the method takes an immutable reference.
 * If you need a mutable reference, change it to `&mut self` instead.
 */
impl MyQueue {

    fn new() -> Self {
        Default::default()
    }

    fn push(&mut self, x: i32) {
        self.input.push(x);
    }

    fn pop(&mut self) -> i32 {
        self.peek();
        self.output.pop().unwrap()
    }

    fn peek(&mut self) -> i32 {
        if self.output.is_empty() {
            while !self.input.is_empty() {
                self.output.push(self.input.pop().unwrap());
            }
        }

        return *self.output.last().unwrap();
    }

    fn empty(&self) -> bool {
        self.input.is_empty() && self.output.is_empty()
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * let obj = MyQueue::new();
 * obj.push(x);
 * let ret_2: i32 = obj.pop();
 * let ret_3: i32 = obj.peek();
 * let ret_4: bool = obj.empty();
 */