bulbs - facebook code lab - partial correct answer

bulbs2.java

public class Solution {
    public int bulbs(List<Integer> a) {
        if(a == null || a.size() == 0)
        return 0; 
        
        int count = 0; 
        while(a !=null && a.size()> 0)
        {
            Integer previous = a.get(0); 
            a.remove(0); 
            
            if(previous.intValue() ==0)
            {
                count ++;
                
                List<Integer> opposite = getOpposite(a); 
                
                a = opposite; 
            }
        }
        
        return count; 
    }
    
    public List<Integer> getOpposite(List<Integer> a)
    {
        List<Integer> list = new ArrayList<Integer>(); 
        if(a== null)
            return list; 
        
        for(int i=0; i< a.size(); i++)
        {
            int val = Integer.valueOf(a.get(i));
            Integer tmp = val == 0? Integer.valueOf(1):Integer.valueOf(0);
            list.add(tmp); 
        }
        
        return list; 
    }
    
}

Here is the error message to work on - from facebook code lab:
Time Complexity
Almost there. Your solution is not well optimized for runtime. Focus your effort in solving the problem in shorter time frame.
Partially Correct Answer. Make your solution more efficient