155.最小栈

class MinStack {
    private Stack<Integer>data;
    private Stack<Integer>help;
    /** initialize your data structure here. */
    public MinStack() {
        data=new Stack<>();
        help=new Stack<>();
    }
    
    public void push(int x) {
        data.add(x);
        if(help.isEmpty()||help.peek()>=x)
            help.add(x);
        else
            help.add(help.peek());
    }
    
    public void pop() {
        if(!data.isEmpty()){
            data.pop();
            help.pop();
        }
    }
    
    public int top() {
        if(!data.isEmpty())
            return data.peek();
        else 
            return -1;
    }
    
    public int getMin() {
        if(!help.isEmpty()){
            return help.peek();
        }
        else return -1;
    }
}

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