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.

• Time : O(1)
  • reverse operation needs o(n) but, after than, we can call pop operation at least n times
  • so total time complex is o(n/n) = o(1)
• Space : O(n)

class MyQueue {
    stack<int> popStack, pushStack;
public:
    MyQueue() {}
    
    void push(int x) {
        pushStack.push(x);
    }
    
    int pop() {
        int res = peek();
        popStack.pop();
        return res;
    }
    
    int peek() {
        if(popStack.empty()) {
            while(!pushStack.empty()) {
                popStack.push(pushStack.top());
                pushStack.pop();
            }
            
        }
        return popStack.top();
    }
    
    bool empty() {
        return pushStack.empty() and popStack.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();
 */