最小栈

class MinStack {
public:
    /** initialize your data structure here. */
    MinStack() {
        
    }

    void push(int val) {

    }
    
    void pop() {
        stack1.pop_back();
    }
    
    int top() {
        return stack1[stack1.size()-1];
    }
    
    int getMin() {
        return minval;
    }
};