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;
    }
}