最小栈

155. 最小栈

1.底层使用链表，每个链表头部节点都存储当前结构里的最小值。

public class MinStack {
    Node head;
    public MinStack() {
    }
    public void push(int val) {
        if (head == null) {
            head = new Node(val, val);
        } else {
            Node node = new Node(val, Math.min(val, head.min), head);
            head = node;
        }
    }
    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;
        public Node(int val, int min, Node next) {
            this.val = val;
            this.min = min;
            this.next = next;
        }
        public Node(int val, int min) {
            this.val = val;
            this.min = min;
        }
    }
}