Found here https://leetcode.com/problems/min-stack/discuss/421143/C++-Beats-90-Cleanest-Code-Simple-(with-Explanation)

MinStack.cpp

struct stackObj {
    int num_;
    int min_; // We track min in the stackObject.
    stackObj* nextItem_;
    stackObj(int num, stackObj* nextItem = nullptr) : num_(num), min_(num), nextItem_(nextItem) {};
}; // The min of a stackObj is initially itself. Later it is compared to the item below it.

class MinStack {
private:
    int size_;
    stackObj* topElement_;

public:
    /** initialize your data structure here. */
    MinStack() {
        size_ = 0;
		topElement = nullptr;
    }
    
    void push(int num) {
        stackObj* newItem = new stackObj(num);
        if (empty()) { // If we start empty, no need to set the min. The min is the first node.
            newItem->nextItem_ = topElement_;
            topElement_ = newItem;
        }
        newItem->nextItem_ = topElement_; // If not empty, set the new stackObj's min
        newItem->min_ = newItem->nextItem_->min_ > num ? num : newItem->nextItem_->min_;
        topElement_ = newItem;
        size_++;
        return;
    }
    
    void pop() {
        if (empty()) return;
        stackObj* pTemp = topElement_;
        topElement_ = topElement_->nextItem_;
        delete pTemp; // Delete item so no memory leak.
        size_--; // Used to check if the stack is empty
        return;
    }
    
    int top() {
        if (empty()) {
            return -1; // Return -1 if empty()
        }
        return topElement_->num_;
    }
    
    int getMin() {
        if (empty()) return -1; // Return -1 if empty()
        return topElement_->min_;
    }
    
    bool empty() const {
        return size_ == 0; // Check whether stack is empty
    }
};