leetcode 225|232. Implement Stack using Queues 232. Implement Queue using Stacks

225. Implement Stack using Queues

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).

class MyStack {public:    /** Initialize your data structure here. */    MyStack() {}        /** Push element x onto stack. */    void push(int x)   //重点就是push进去的时候花点时间，把前面所有的重新push一次    {        int size = que.size();        que.push(x);        for (int i = 0; i < size; i++)        {            que.push(que.front());            que.pop();        }    }        /** Removes the element on top of the stack and returns that element. */    int pop()     {        int key = que.front();        que.pop();        return key;    }        /** Get the top element. */    int top()     {        return que.front();    }        /** Returns whether the stack is empty. */    bool empty() {        return que.empty();        }private:    queue<int> que;};/** * 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(); * bool param_4 = obj.empty(); */

232. 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).

使用两个stack

class MyQueue {public:    /** Initialize your data structure here. */    MyQueue() {            }        /** Push element x to the back of queue. */    void push(int x)     {        while (!sk.empty())            {            tmp.push(sk.top());            sk.pop();        }        sk.push(x);        while (!tmp.empty())        {            sk.push(tmp.top());            tmp.pop();        }    }        /** Removes the element from in front of queue and returns that element. */    int pop()     {        int ret = sk.top();        sk.pop();            return ret;    }        /** Get the front element. */    int peek()     {        return sk.top();    }        /** Returns whether the queue is empty. */    bool empty()     {        return sk.empty();    }private:    stack<int> sk;    stack<int> tmp;};/** * 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(); * bool param_4 = obj.empty(); */