• [x] ##### Min Stack这个题目，用了Node，这个思想很厉害！！！！
• [x] ##### 用Node, 通过一个linked list of nodes, 完成remove 操作，用head一直指向栈顶，
• [x] ##### 并且用一个min永远保存当前最小的min！！！！

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• getMin() -- Retrieve the minimum element in the stack.

Example:

MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin();   -->Returns -3.
minStack.pop();
minStack.top();      ->Returns 0.
minStack.getMin();   -->Returns -2.//-3已经被pop出去了。怎么更新min value in stack

class MinStack {

    /** initialize your data structure here. */
    private Node head;
    public MinStack() {

    }

    public void push(int x) {
        if (head == null) {
            head = new Node(x, x);
        }else{
            head = new Node(x, Math.min(x, 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;
        }
    }

}


/**
 * 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();
 */