解题思路

每次如果当前值比最小值还要小，则将第二小的值入栈，这样即使最小值出栈后也有最小值信息
代码

class MinStack {
    int min = Integer.MAX_VALUE;
    Stack<Integer> stack = new Stack<Integer>();
    public void push(int x) {
        // only push the old minimum value when the current 
        // minimum value changes after pushing the new value x
        if(x <= min){          
            stack.push(min);
            min=x;
        }
        stack.push(x);
    }
    public void pop() {
        // if pop operation could result in the changing of the current minimum value, 
        // pop twice and change the current minimum value to the last minimum value.
        if(stack.pop() == min) min=stack.pop();
    }
    public int top() {
        return stack.peek();
    }
    public int getMin() {
        return min;
    }
}