232. 用栈实现队列 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.

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).

public class MyQueue {
    public Stack<int> stack;

    public MyQueue() {
        stack = new Stack<int>();
    }

    public void Push(int x) {
        stack.Push(x);
    }

    public int Pop() {
        Stack<int> tempStack = new Stack<int>();
        int count = stack.Count();
        for (int i = 0; i < count; i++) {
            tempStack.Push(stack.Pop());
        }
        int peek = tempStack.Pop();
        stack.Clear();
        count = tempStack.Count();
        for (int i = 0; i < count; i++) {
            stack.Push(tempStack.Pop());
        }
        return peek;
    }

    public int Peek() {
        int[] arr = stack.ToArray();
        return arr[stack.Count - 1];
    }

    public bool Empty() {
        return this.stack.Count == 0;
    }
}

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