LeetCode Text Justification

public class Solution {
    public List<String> fullJustify(String[] words, int maxWidth) {
        List<String> res = new ArrayList<String>();
        if(words == null || words.length == 0){
            return res;
        }
        int len = words.length;
        int count = 0;
        int last = 0;
        for(int i = 0; i<len; i++){
            if(count + (i-last) + words[i].length() > maxWidth){
                int spaceNum = 0;
                int extraSpace = 0;
                if(i-last-1>0){ //如果本行不止一个词，spaceNum和extraSpace才会update
                    spaceNum = (maxWidth-count)/(i-last-1);
                    extraSpace = (maxWidth-count)%(i-last-1);
                }
                
                StringBuilder sbPerLine = new StringBuilder();
                for(int j = last; j<i; j++){
                    sbPerLine.append(words[j]);
                    if(j<i-1){ //每一行最后一个词后面不用加空格,若是本行只有一个词，下面处理
                        for(int k = 0; k<spaceNum; k++){
                            sbPerLine.append(" ");
                        }
                        if(extraSpace > 0){
                            sbPerLine.append(" ");
                            extraSpace--;
                        }
                        
                    }
                }
                
                //如果本行只有一个词
                for(int j = sbPerLine.length(); j<maxWidth; j++){
                    sbPerLine.append(" ");
                }
                
                res.add(sbPerLine.toString());
                count = 0;
                last = i;
            }
            count += words[i].length();
        }
        //处理最后一行，若是只有一行且长度正好等于maxWidth也在下面处理
        StringBuilder lastLine = new StringBuilder();
        for(int i = last; i<len; i++){
            lastLine.append(words[i]);
            if(lastLine.length()<maxWidth){
                lastLine.append(" ");
            }
        }
        for(int i = lastLine.length(); i<maxWidth; i++){
            lastLine.append(" ");
        }
        res.add(lastLine.toString());
        
        return res;
    }
}