232.MyQueue.cs

// 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

public class MyQueue {
    Stack<int> st1;
    Stack<int> st2;
    public MyQueue() {
        st1 = new Stack<int>();  
        st2 = new Stack<int>();        
    }
    
    public void Push(int x) {
        st1.Push(x);
    }
    
    public int Pop() {
        while(st1.Count>0){
            st2.Push(st1.Pop());
        }
        int res = st2.Pop();
        while(st2.Count>0){
            st1.Push(st2.Pop());
        }
        return res;
    }
    
    public int Peek() {
        while(st1.Count>0){
            st2.Push(st1.Pop());
        }
        int res = st2.Peek();
        while(st2.Count>0){
            st1.Push(st2.Pop());
        }
        return res;
    }
    
    public bool Empty() {
        
        return st1.Count == 0;
    }
}

/**
 * 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();
 */