代码随想录算法训练营 day 10: 232.用栈实现队列 | 225. 用队列实现栈

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

 

Constraints:

• 1 <= x <= 9
• At most 100 calls will be made to push, pop, peek, and empty.
• All the calls to pop and peek are valid.

用栈实现队列需要两个栈，一个进一个出。

代码：

class MyQueue {

    public MyQueue() {
        istack = new Stack<Integer>();
        ostack = new Stack<Integer>();


    }
    
    public void push(int x) {
        istack.push(x);
    }
    
    public int pop() {
        if(ostack.empty()) {
            while(!istack.empty()) {
                Integer top = istack.pop();
                ostack.push(top);
            }

            return ostack.pop();
        }
        else {
            return ostack.pop();
        }
    }
    
    public int peek() {
        if(ostack.empty()) {
            while(!istack.empty()) {
                Integer top = istack.pop();
                ostack.push(top);
            }

            return ostack.peek();
        }
        else {
            return ostack.peek();
        }        
        
    }
    
    public boolean empty() {
        return (istack.empty() && ostack.empty());
    }

    Stack<Integer> istack;
    Stack<Integer> ostack;
}

/**
 * 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();
 * boolean param_4 = obj.empty();
 */

225. Implement Stack using Queues

Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push, top, pop, and empty).

Implement the MyStack class:

• void push(int x) Pushes element x to the top of the stack.
• int pop() Removes the element on the top of the stack and returns it.
• int top() Returns the element on the top of the stack.
• boolean empty() Returns true if the stack is empty, false otherwise.

Notes:

• You must use only standard operations of a queue, which means that only push to back, peek/pop from front, size and is empty operations are valid.
• Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue) as long as you use only a queue's standard operations.

 

Example 1:

Input
["MyStack", "push", "push", "top", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 2, 2, false]

Explanation
MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // return 2
myStack.pop(); // return 2
myStack.empty(); // return False

 

Constraints:

• 1 <= x <= 9
• At most 100 calls will be made to push, pop, top, and empty.
• All the calls to pop and top are valid.

 

Follow-up:  Can you implement the stack using only one queue?

这题使用两个queue比起使用一个queue并没有增加效率，TC，SC全都一样。用一个队列的话，在pop的时候要把队列的前n-1个元素依次pop并添加到队尾。然后pop

Deque的系统API很不熟，要多练。

代码：

class MyStack {

    public MyStack() {
        mq = new ArrayDeque<Integer>();
    }
    
    public void push(int x) {
        mq.addLast(x);
    }
    
    public int pop() {
        int size = mq.size();


        while(size-- > 1) {
            Integer cur = mq.peekFirst();
            mq.addLast(cur);
            mq.pollFirst();
        }

        return mq.pollFirst();
    }
    
    public int top() {
        return mq.peekLast();
    }
    
    public boolean empty() {
        return mq.isEmpty();
    }

    Deque<Integer> mq;
}

/**
 * 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();
 */