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).
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思路： 用两个栈来实现队列。先进先出变成先进后出，大概意思就是，把第一个栈里面的东西，都倒腾到第二个里面去，顺序反过来，就可以了。 具体还有一些细节可以优化。
class MyQueue { // Push element x to the back of queue. Stack<Integer> tmp = new Stack<Integer>(); Stack<Integer> value = new Stack<Integer>(); public void push(int x) { if (!tmp.isEmpty()) tmp.push(x); else{ while(!value.isEmpty()) tmp.push(value.pop()); tmp.push(x); } } // Removes the element from in front of queue. public void pop() { if(value.isEmpty() && tmp.isEmpty()) return; if(!value.isEmpty()) value.pop(); else{ while(!tmp.isEmpty()) value.push(tmp.pop()); value.pop(); } } // Get the front element. public int peek() { if(value.isEmpty() && tmp.isEmpty()) return 0; if(!value.isEmpty()) return value.peek(); else{ while(!tmp.isEmpty()) value.push(tmp.pop()); return value.peek(); } } // Return whether the queue is empty. public boolean empty() { return tmp.isEmpty() && value.isEmpty(); } }