思路

1. 维护两个栈，一个存数据的普通栈，一个存最小值的单调栈
2. 当当前最小值就是数据栈栈顶时，同时抛出两个栈的栈顶
3. 当插入值x小于单调栈栈顶时，将x压入单调栈

C++代码

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

    }

    void push(int x) {
        if(stack_min.empty() || stack_min.top() >= x) stack_min.push(x);
        stack_value.push(x);
    }

    void pop() {
        if(stack_min.top() == stack_value.top()) stack_min.pop();
        stack_value.pop();
    }

    int top() {
        return stack_value.top();
    }

    int getMin() {
        return stack_min.top();
    }
};

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack obj = new MinStack();
 * obj.push(x);
 * obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.getMin();
 */