leetcode------Min Stack

标题：	Min Stack
通过率：	15.2%
难度：	简单

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.

本题如果直接用java内置的stack，那么迎刃而解，出这个题的目的就是不让用把。我直接用了模拟栈，头插入链表去解决这个问题，题目相对比较简单，实现的时候做好为空判断和最小值的问题，看代码就能看出来最小值的问题。代码如下：

class MinStack {
    Node top = null;

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

    public void pop() {
        top = top.next;
 
    }

    public int top() {
        return top == null ? 0 : top.val;
    }

    public int getMin() {
        return top == null ? 0 : top.min;
    }
}

class Node {
    int val;
    int min;
    Node next;

    public Node(int val) {
        this.val = val;
    }
}