# 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 top, peek/pop from top, size, 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).

# Solution

Approach 1: Two Stacks: Push - O(n) per operation, Pop - O(1) per operation.

Approach 2: Two Stacks: Push - O(1) per operation, Pop - Amortized O(1) per.

# Code (Python)

Approach 1:

Approach 2:

# Code (C++)

Approach 1:

/**
 * 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<int> s1;
    stack<int> s2;
public:
    // Initialize your data structure here.
    MyQueue() {
    }
    
    // Push element x to the back of queue.
    void push(int x) {
        while (!s1.empty())
        {
            s2.push(s1.top());
            s1.pop();
        }
        s1.push(x);
        while (!s2.empty())
        {
            s1.push(s2.top());
            s2.pop();
        }
    }
    
    // Removes the element from in front of queue and returns that element.
    int pop() {
        int popVal = s1.top();
        s1.pop();
        return popVal;
    }
    
    // Get the front element.
    int peek() {
        return s1.top();
    }
    
    // Returns whether the queue is empty.
    bool empty() {
        return s1.empty();
    }
};

Approach 2:

class MyQueue {
private:
    stack<int> s1;
    stack<int> s2;
    int front;
public:
    // Initialize your data structure here.
    MyQueue() {
    }
    
    // Push element x to the back of queue.
    void push(int x) {
        if (s1.empty())
            front = x;
        s1.push(x);
    }
    
    // Removes the element from in front of queue and returns that element.
    int pop() {
        if (s2.empty())
        {
            while (!s1.empty())
            {
                s2.push(s1.top());
                s1.pop();
            }
        }
        int popVal = s2.top();
        s2.pop();
        return popVal;
    }
    
    // Get the front element.
    int peek() {
        if (!s2.empty())
            return s2.top();
        return front;
    }
    
    // Returns whether the queue is empty.
    bool empty() {
        return s1.empty() && s2.empty();
    }
};