50.Min Stack-最小栈

class MinStack {
    private List<Integer> stack;
    private List<Integer> assit;  //辅助栈
    /** initialize your data structure here. */
    public MinStack() {
        this.stack=new ArrayList<>();
        this.assit=new ArrayList<>();  
    }
    public void push(int x) {
        stack.add(x);
        if (assit.isEmpty()){
            assit.add(x);
        }else if(x< assit.get(assit.size()-1)){
            assit.add(x);
        }else{
            assit.add(assit.get(assit.size()-1));
        }
    }
    public void pop() {
        stack.remove(stack.size()-1);
        assit.remove(assit.size()-1);
    }
    public int top() {
        return stack.get(stack.size()-1);
    }
    public int getMin() {
        return assit.get(assit.size()-1);
    }
}