155.最小栈¶

输入：
["MinStack","push","push","push","getMin","pop","top","getMin"]
[[],[-2],[0],[-3],[],[],[],[]]

输出：
[null,null,null,null,-3,null,0,-2]

解释：
MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin();   --> 返回 -3.
minStack.pop();
minStack.top();      --> 返回 0.
minStack.getMin();   --> 返回 -2.

class MinStack {

    /** initialize your data structure here. */
    LinkedList<Integer> stack = new LinkedList<>();
    LinkedList<Integer> minStack = new LinkedList<>();
    public MinStack() {
        minStack.push(Integer.MAX_VALUE);
    }
    //主栈添加一个元素 辅助栈同时添加一个当前最小元素
    public void push(int val) {
        stack.push(val);
        //还可以优化一下，不用每次都存
        minStack.push(Math.min(val,minStack.peek()));
    }
    //同时弹出
    public void pop() {
        stack.pop();
        minStack.pop();
    }

    public int top() {
        return stack.peek();
    }
    //本题难点是如何获取栈中的最小元素
    public int getMin() {
        return minStack.peek();
    }
}

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack obj = new MinStack();
 * obj.push(val);
 * obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.getMin();
 */