Given a sequence of words, check whether it forms a valid word square.A sequence of words forms a valid word square if the kth row and column read the exact same string, where 0 ≤ k < max(numRows, numColumns).Note:The number of words given is at least 1 and does not exceed 500.Word length will be at least 1 and does not exceed 500.Each word contains only lowercase English alphabet a-z.

这题看起来简单但是其实很容易写错

本来想j 不从0开始，而是从 i+1开始检查，可以节省时间，但是一些为Null的情况会使讨论很复杂

比如

[  "abcd",  "bnrt",  "crm",  "dtz"] 第三行检查到m就结束了，因为j
    
     > 结构，两两比较，最好保证其中一个始终不为null，另一个为null就报错，这样可以省掉很多讨论情况 然后要保证一个始终不为null，就需要j
    

1 public class Solution { 2     public boolean validWordSquare(List
     
       words) { 3         if(words == null || words.size() == 0){ 4             return true; 5         } 6          7         for (int i=0; i