[leetcode]Text Justification

这道题果然不容易写对啊。首先为了写对，我写完以后对着代码又检查了一阵，挺好，而且发现自己偶尔写点注释也便于自己理解。

然后居然超时了，主要原因就是计算长度为n的空字符串花时间太多，就用了个数组一开始先算好存起来。这里用new String[L+1]主要为了避免L为0的情况。

public class Solution {

    public ArrayList<String> fullJustify(String[] words, int L) {

        // Start typing your Java solution below

        // DO NOT write main() function

        ArrayList<String> result = new ArrayList<String>();

        String[] spaces = new String[L+1];

        StringBuilder builder = new StringBuilder();

        for (int i = 0; i < L+1; i++) {

            spaces[i] = builder.toString();

            builder.append(' ');

        }

        

        int len = words.length;

        

        int i = 0;

         while (i < len) {

             int j = i;

             int total = words[i].length();

             while (true) { 

                 j++;

                 if (j == len) break;

                 int w = words[j].length();

                 if (total + w + 1 > L) break;

                 total += w + 1;                

             }

             int left = L - total;

             int c = j - i;

             if (c == 1) { // single word

                 StringBuilder sb = new StringBuilder(words[i]);

                 sb.append(spaces[left]);

                 result.add(sb.toString());

             }

             else { // multi-word

                 if (j == len) { // last line

                     StringBuilder sb = new StringBuilder(words[i]);

                     for (int x = i+1; x < j; x++) {

                         sb.append(' ');

                         sb.append(words[x]);

                     }

                     sb.append(spaces[left]);

                     result.add(sb.toString());

                 }

                 else {

                     int k = left / (c - 1); // extra ' ' for every gap

                     int l = left - k * (c - 1); // count for first gaps with extra '' 

                     StringBuilder sb = new StringBuilder(words[i]);

                     for (int x = i+1; x < j; x++) {

                         if (l > 0) {

                             sb.append(' ');

                             l--;

                         }

                         sb.append(' ');

                         sb.append(spaces[k]);

                         sb.append(words[x]);

                     }

                     result.add(sb.toString());

                 }

             }

             i = j;

         }        

         return result; 

    }

}