155. 最小栈 Java版

class MinStack {

    private Node head;

    public MinStack() {

    }
    
    public void push(int val) {

        if(head == null)
            head = new Node(val,val);
        else
            head = new Node(val , Math.min(val,head.min) , head);
    }
    
    public void pop() {
        head = head.next;
    }
    
    public int top() {
        return head.val;
    }
    
    public int getMin() {
        return head.min;
    }

    private class Node {
        int val;
        int min;
        Node next;
        
        private Node(int val, int min) {
            this(val, min, null);
        }
        
        private Node(int val, int min, Node next) {
            this.val = val;
            this.min = min;
            this.next = next;
        }
    }
}