力扣-155题 最小栈（C++）- 单调栈

题目链接：https://leetcode-cn.com/problems/min-stack/
题目如下：

class MinStack {
public:
    /** initialize your data structure here. */
    stack<int> stk,minstk;
    MinStack() {

    }
    
    void push(int val) {
        stk.push(val);
        if(minstk.size()!=0) minstk.push(min(minstk.top(),val));
        else minstk.push(val);
    }
    
    void pop() {
        stk.pop();
        minstk.pop();
    }
    
    int top() {
        if(stk.size()!=0) return stk.top();
        else return -1;
    }
    
    int getMin() {
        if(minstk.size()!=0) return minstk.top();
        else return -1;
    }
};

/**
 * 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();
 */