68. Text Justification

Given an array of words and a length L, format the text such that each line has exactly L characters and is fully (left and right) justified.

You should pack your words in a greedy approach; that is, pack as many words as you can in each line. Pad extra spaces ‘ ‘ when necessary so that each line has exactly L characters.

Extra spaces between words should be distributed as evenly as possible. If the number of spaces on a line do not divide evenly between words, the empty slots on the left will be assigned more spaces than the slots on the right.

For the last line of text, it should be left justified and no extra space is inserted between words.

For example,
words: ["This", "is", "an", "example", "of", "text", "justification."]
L: 16.

Return the formatted lines as:

[
   "This    is    an",
   "example  of text",
   "justification.  "
]

public class Solution {
    public List<String> fullJustify(String[] words, int L) {
      List<String> res = new ArrayList<>();
      if(words == null || words.length == 0 || L==0) {
          res.add("");
          return res;
      }
      int count = 0;
      int last = 0;
      for(int i = 0 ; i < words.length ; i++){
          if(count + words[i].length() + i - last > L){ //count 计数加该单词的长度＋ 空格数 last是上一行最后一个单词
              // fail to add new word to this line
              int space = 0;
              int extraSpace = 0;
              if(i - last - 1 > 0){//因为fail，空格数要减1
                  space = (L - count) / (i - last - 1);
                  extraSpace = (L - count) % (i - last - 1);
              }
              StringBuilder str = new StringBuilder();
              for(int j = last ; j < i ; j++){   // 填单词和空格
                  str.append(words[j]);
                  if(j < i-1){  //最后一个单词后不用填空格
                      for(int k = 0 ; k < space; k++){
                          str.append(" ");
                          if(extraSpace > 0)
                            str.append(" ");
                          extraSpace --;
                      }
                  }
              }

              for(int j = str.toString().length(); j < L; j ++){
                   str.append(" ");
              }
              res.add(str.toString());
              last = i;
              count = 0;
          }
          count += words[i].length();
      }

      //处理最后一行 只有一个word
      StringBuilder str = new StringBuilder();
      for(int  i = last ; i < words.length ; i++){
          str.append(words[i]);
          if(words[i].length() < L)
            str.append(" ");
      }
      for(int j = str.toString().length(); j < L; j ++){
          str.append(" ");
      }
      res.add(str.toString());

     return res;
    }
}

Note: Each word is guaranteed not to exceed L in length.