手把手带你了解C++最小栈

```cpp
class MinStack {
public:
    /** initialize your data structure here. */
    //创建两个栈
    stack<int> datastack; //数据栈
    stack<int> sortedstack; //有序栈，栈底最大，栈顶最小，有<=栈顶的元素才push
    MinStack() {
    }
    void push(int val) {
        datastack.push(val); //将值push到数据栈
        //有序栈
        //当有序栈为空 或 值小于等于有序栈的栈顶 就push
        if(sortedstack.empty()||val<=sortedstack.top()) //sortedstack==sortedstack.empty() 错误
            sortedstack.push(val);
    }
    void pop() {
        if(datastack.top()==sortedstack.top())  //数据栈的栈顶 和 有序栈的栈顶 相同， 有序栈出栈
            sortedstack.pop();
        datastack.pop();  //数据栈一直在出栈
    }         
    int top() {
        return datastack.top();
    }
    int getMin() {
        return sortedstack.top();
    }
};
/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack* obj = new MinStack();
 * obj->push(val);
 * obj->pop();
 * int param_3 = obj->top();
 * int param_4 = obj->getMin();
 */