Text Justification -- LeetCode

public ArrayList<String> fullJustify(String[] words, int L) {
    ArrayList<String> res = new ArrayList<String>();
    if(words==null || words.length==0)
        return res;
    int count = 0;
    int last = 0;
    for(int i=0;i<words.length;i++)
    {
        if(count+words[i].length()+(i-last)>L)
        {
            int spaceNum = 0;
            int extraNum = 0;
            if(i-last-1>0)
            {
                spaceNum = (L-count)/(i-last-1);
                extraNum = (L-count)%(i-last-1);
            }
            StringBuilder str = new StringBuilder();
            for(int j=last;j<i;j++)
            {
                str.append(words[j]);
                if(j<i-1)
                {
                    for(int k=0;k<spaceNum;k++)
                    {
                        str.append(" ");
                    }
                    if(extraNum>0)
                    {
                        str.append(" ");
                    }
                    extraNum--;
                }
            }
            for(int j=str.length();j<L;j++)
            {
                str.append(" ");
            }       
            res.add(str.toString());
            count=0;
            last=i;
        }
        count += words[i].length();
    }
    StringBuilder str = new StringBuilder();
    for(int i=last;i<words.length;i++)
    {
        str.append(words[i]);
        if(str.length()<L)
            str.append(" ");
    }
    for(int i=str.length();i<L;i++)
    {
        str.append(" ");
    }
    res.add(str.toString());
    return res;
}