Min Stack，minstack

Min Stack，minstack

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

• push(x) – Push element x onto stack.
• pop() – Removes the element on top of the stack.
• top() – Get the top element.
• getMin() – Retrieve the minimum element in the stack.

設計並實現一個stack,但是這個stack要隨時都可以返回一個最小值。可以再申請一個stack儲存當前stack中的最小值，同樣的思路也可以解決Min Stack和Max Stack的問題。
C++ Code:

class MinStack {private:          vector<int> mStack;         vector<int> mMinStack;         int mMinValue = 0;public:    void push(int x) {        if (mStack.size() == 0)            mMinValue = x;        else            mMinValue = mMinValue < x ? mMinValue : x;        mStack.push_back(x);        mMinStack.push_back(mMinValue);    }    void pop() {        mStack.pop_back();        mMinStack.pop_back();        mMinValue = getMin();    }    int top() {        if (mStack.size() > 0)            return mStack[mStack.size() - 1];        else            return -1;    }    int getMin() {        if (mMinStack.size() > 0)            return mMinStack[mMinStack.size() - 1];        else            return -1;    }};