思路：

使用2個棧，其中一個棧作為普通棧使用，另一個只把最小的元素入棧。

入棧：如果元素小於或等於最小元素棧頂，則入最小元素棧

出棧：如果元素等於最小元素棧頂，則最小元素出棧

Code：

class MinStack {
    private Stack<Integer> innerStack;
    private Stack<Integer> minStack;
    /** initialize your data structure here. */
    public MinStack() {
        innerStack = new Stack<Integer>();
        minStack = new Stack<Integer>();
    }
    
    public void push(int x) {
        innerStack.push(x);
        //add to queue
        if (minStack.isEmpty() || x <= minStack.peek()) {
            minStack.push(x);
        }
    }
    
    public Integer pop() {
        if (!innerStack.empty()) {
            Integer peek = innerStack.pop();
            if (peek.equals(minStack.peek())) {
                minStack.pop();
            }
            return peek;
        }
        return null;
    }
    
    public Integer top() {
        if (!innerStack.empty()) {
            return innerStack.peek();
        }
        return null;
    }
    
    public Integer getMin() {
        if (!minStack.empty()) {
            return minStack.peek();
        }
        return null;
    }
}