155. 最小栈

class MinStack {
    //辅助栈的使用
    //1:当一个元素要入栈时，我们取当前辅助栈的栈顶存储的最小值，与当前元素比较得出最小值，将这个最小值插入辅助栈中；
    //2:当一个元素要出栈时，我们把辅助栈的栈顶元素也一并弹出
    //3:在任意一个时刻，栈内元素的最小值就存储在辅助栈的栈顶元素中
    Stack<Integer> stack;
    Stack<Integer> minStack;
    public MinStack() {
        stack = new Stack<Integer>();
        minStack = new Stack<Integer>();
        minStack.push(Integer.MAX_VALUE);
    }
    public void push(int val) {
        stack.push(val);
        minStack.push(Math.min(minStack.peek(),val));
    }
    public void pop() {
        stack.pop();
        minStack.pop();
    }
    public int top() {
        return stack.peek();
    }
    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();
 */