implement_queue_using_stacks.cpp

/*
232. Implement Queue using Stacks
Easy

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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).
*/

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

/**
 * 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();
 */