155.Min Stack
https://leetcode.com/problems/min-stack/
s2存储最小值，s1存储栈本身。

class MinStack {
private:
    stack<int> s1;
    stack<int> s2;
public:
    void push(int x) {
        s1.push(x);
        if (s2.empty() || x <= getMin())  s2.push(x);       
    }
    void pop() {
        if (s1.top() == getMin())  s2.pop();
        s1.pop();
    }
    int top() {
        return s1.top();
    }
    int getMin() {
        return s2.top();
    }
};

FB面经：Min Queue
minque存储最小值，

class MinQueue{
    queue<int> que;
    deque<int> minque;
public:
    void push(int x) {
        if(que.size() == 0) {
            que.push(x);
            minque.push_back(x);
        } else {
            que.push(x);
            while(!minque.empty() && minque.back() > x) {
                minque.pop_back();
            }
            minque.push_back(x);
        }
    }
    void pop() {
        if(que.size() == 0) return;
        if(que.front() == minque.front())
            minque.pop_front();
        que.pop();
    }
    int front() {
        return que.front();
    }
    int getMin(){
        return minque.front();
    }
};    

232.Implement Queue using Stacks

class Queue {
    stack<int> input, output;
public:

    void push(int x) {
        input.push(x);
    }

    void pop(void) {
        peek();
        output.pop();
    }

    int peek(void) {
        if (output.empty())
            while (input.size())
                output.push(input.top()), input.pop();
        return output.top();
    }

    bool empty(void) {
        return input.empty() && output.empty();
    }
};    

225.implement Stack using Queues

class Stack {
    queue<int> que;
public:
    // Push element x onto stack.
    void push(int x) {
        que.push(x);
        int n = que.size() - 1;
        for(int i = 0; i < n; i++) {
            que.push(que.front());
            que.pop();
        }
    }

    // Removes the element on top of the stack.
    void pop() {
        que.pop();
    }

    // Get the top element.
    int top() {
        return que.front();
    }

    // Return whether the stack is empty.
    bool empty() {
        return que.empty();
    }
};