Leetcode
2020.05.05 16:44

225. Implement Stack using Queues

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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.

Example:

```MyStack stack = new MyStack();

stack.push(1);
stack.push(2);
stack.top();   // returns 2
stack.pop();   // returns 2
stack.empty(); // returns false```

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).

```class MyStack {
List<Integer> list;

/** Initialize your data structure here. */
public MyStack() {
list = new ArrayList<>();
}

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

/** Removes the element on top of the stack and returns that element. */
public int pop() {
return list.remove(list.size()-1);
}

/** Get the top element. */
public int top() {
return list.get(list.size()-1);
}

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

/**
* Your MyStack object will be instantiated and called as such:
* MyStack obj = new MyStack();
* obj.push(x);
* int param_2 = obj.pop();
* int param_3 = obj.top();
* boolean param_4 = obj.empty();
*/```

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