Solution：
思路：
Time Complexity: O(N) Space Complexity: O(N)
Solution Code：
public class Solution {
public boolean validWordSquare(List<String> words) {
if(words == null || words.size() == 0){
return true;
}
int n = words.size();
for(int i=0; i<n; i++){
for(int j=0; j<words.get(i).length(); j++){
if(j >= n || words.get(j).length() <= i || words.get(j).charAt(i) != words.get(i).charAt(j))
return false;
}
}
return true;
}
}
