[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; 
    }
}