Implement Stack using Queues ——LeetCode

Implement the following operations of a stack using queues.

push(x) -- Push element x onto stack.

pop() -- Removes the element on top of the stack.

top() -- Get the top element.

empty() -- Return whether the stack is empty.

Notes:

You must use only standard operations of a queue -- which means only

push to back

,

peek/pop from front

,

size

, and

is empty

operations are valid.

Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.

You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

题目大意：用队列，实现栈。

class MyStack {

List<Integer> stack = new ArrayList<>();

// Push element x onto stack.
public void push(int x) {
stack.add(x);
}

// Removes the element on top of the stack.
public void pop() {
if(!empty()){
stack.remove(stack.size()-1);
}
}

// Get the top element.
public int top() {
if(!empty()){
return stack.get(stack.size()-1);
}
return -1;
}

// Return whether the stack is empty.
public boolean empty() {
return stack.size()==0;
}
}