[LeetCode] Min Stack

class MinStack {
    stack<int> s;
    stack<int> s_min;
    
public:
    void push(int x) {
        s.push(x);
        if (s_min.empty() || x <= s_min.top()) {
            s_min.push(x);
        }
    }

    void pop() {
        if (s.top() == s_min.top()) {
            s_min.pop();
        }
        s.pop();
    }

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

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

struct node {
    int val;
    node *next;
    node *min;
    node():val(-1), next(NULL), min(NULL){}
    node(int v):val(v), next(NULL), min(NULL){}
};

class MinStack {
    node head;
    
public:
    void push(int x) {
        node *top, *pre;
        if(head.next == NULL) {
            head.next = new node(x);
            top = head.next;
            top->min = top;
        } else {
            pre = head.next;
            head.next = new node(x);
            top = head.next;
            top->next = pre;
            if (x <= pre->min->val) {
                top->min = top;
            } else {
                top->min = pre->min;
            }
        }
    }

    void pop() {
        if (head.next == NULL) return;
        node *tmp = head.next;
        head.next = tmp->next;
        delete tmp;
    }

    int top() {
        return head.next->val;
    }

    int getMin() {
        return head.next->min->val;
    }
};