Problem

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.

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).

Example

MyQueue queue = new MyQueue();

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

Code

static int var = [](){
    std::ios::sync_with_stdio(false);
    cin.tie(NULL);
    return 0;
}();
class MyQueue {
private:
    stack<int> s;
public:
    /** Initialize your data structure here. */
    MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        s.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        stack<int> temp;
        int s_size = s.size();
        for(int i=0;i<s_size-1;i++){
            temp.push(s.top());
            s.pop();
        }
        int ret = s.top();
        s.pop();
        int t_size = temp.size();
        for(int i=0;i<t_size;i++){
            s.push(temp.top());
            temp.pop();
        }
        return ret;
    }
    
    /** Get the front element. */
    int peek() {
        stack<int> temp;
        int s_size = s.size();
        for(int i=0;i<s_size-1;i++){
            temp.push(s.top());
            s.pop();
        }
        int ret = s.top();
        int t_size = temp.size();
        for(int i=0;i<t_size;i++){
            s.push(temp.top());
            temp.pop();
        }
        return ret;
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return s.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();
 */

Result

232. Implement Queue using Stacks.png