953. Verifying an Alien Dictionary

Solution.java

class Solution {
  public boolean isAlienSorted(String[] words, String order) {
    if (words == null || words.length == 0 || order == null || order.length() == 0) {
      return false;
    }
    Map<Character, Integer> orderRule = new HashMap<>();
    for (int i = 0; i < order.length(); ++i) {
      char ch = order.charAt(i);
      orderRule.put(ch, i);
    }
    
    for (int i = 0; i < words.length - 1; ++i) {
      String s1 = words[i];
      String s2 = words[i + 1];
      int j = 0;
      while (j < s1.length() && j < s2.length() && s1.charAt(j) == s2.charAt(j)) {
        ++j;
      }
      if (j < s1.length() && j < s2.length()) {
        if (orderRule.get(s1.charAt(j)) > orderRule.get(s2.charAt(j))) {
          return false;
        }
      } else {
        if (j < s1.length() && j >= s2.length()) return false; // "abcdef" vs. "abc"
      }
    }
    return true;
  }
}