953. Verifying an Alien Dictionary

QUESTION:

In an alien language, surprisingly they also use english lowercase letters, but possibly in a different order. The order of the alphabet is some permutation of lowercase letters.

Given a sequence of words written in the alien language, and the order of the alphabet, return true if and only if the given words are sorted lexicographicaly in this alien language.

Example 1:

Input: words = ["hello","leetcode"], order = "hlabcdefgijkmnopqrstuvwxyz"
Output: true
Explanation: As 'h' comes before 'l' in this language, then the sequence is sorted.

Example 2:

Input: words = ["word","world","row"], order = "worldabcefghijkmnpqstuvxyz"
Output: false
Explanation: As 'd' comes after 'l' in this language, then words[0] > words[1], hence the sequence is unsorted.

Example 3:

Input: words = ["apple","app"], order = "abcdefghijklmnopqrstuvwxyz"
Output: false
Explanation: The first three characters "app" match, and the second string is shorter (in size.) According to lexicographical rules "apple" > "app", because 'l' > '∅', where '∅' is defined as the blank character which is less than any other character (More info).

Note:

  1. 1 <= words.length <= 100
  2. 1 <= words[i].length <= 20
  3. order.length == 26
  4. All characters in words[i] and order are english lowercase letters.

EXPLANATION:

两种方式，明显第二种其实是更优雅的。

首先需要建立一个新的map来记录新的字符串的位置。map[字符]=位置

第二步再计算每个字符串是否是按照位置来的。map[需要确认的字符]得到新map的位置，然后比较两者的位置顺序。

SOLUTION:

class Solution {
    public boolean isAlienSorted(String[] words, String order) {
        char[] orders = order.toCharArray();
        for(int i =1;i<words.length;i++){
            if(!isAlienSortedHelper(words[i-1],words[i],orders)) return false;
        }
        return true;
    }
    
    public static boolean isAlienSortedHelper(String A,String B,char[] order){
        char[] charsA = A.toCharArray();
        char[] charsB = B.toCharArray();
        int index = 0;
        while (index<charsA.length||index<charsB.length){
            char tmpA = ' ';
            char tmpB = ' ';
            if(index<=charsA.length-1)
                tmpA = charsA[index];
            if(index<=charsB.length-1)
                tmpB = charsB[index];
            if(tmpA==' '&&tmpB!=' '){
                return true;
            }else if(tmpA!=' '&&tmpB==' '){
                return false;
            }else{
                int indexA =-1;
                int indexB =-1;
                for(int i = 0;i<order.length;i++){
                    if(order[i]==tmpA)
                        indexA=i;
                    if(order[i]==tmpB)
                        indexB=i;
                    if(indexA!=-1&&indexB!=-1&&indexA<indexB) return true;
                    if(indexA!=-1&&indexB!=-1&&indexA>indexB) return false;
                    if(indexA!=-1&&indexB!=-1&&indexA==indexB) break;
                }
            }
            index++;
        }
        return true;
    }
}

class Solution {
    int[] mapping = new int[26];
    public boolean isAlienSorted(String[] words, String order) {
        for (int i = 0; i < order.length(); i++) {
            mapping[order.charAt(i) - 'a'] = i;
        }
        for (int i = 0; i < words.length - 1; i++) {
            if (!small(words[i], words[i+1])) return false;
        }
        return true;
    }
    public boolean small(String s1, String s2) {
        int n = s1.length(), m = s2.length();
        for (int i = 0; i < n && i < m; i++) {
            //这里无论是大于还是小于都能马上知道是不是正确sort，
            if (mapping[s1.charAt(i) - 'a'] != mapping[s2.charAt(i) - 'a']) {
                return mapping[s1.charAt(i) - 'a'] < mapping[s2.charAt(i) - 'a'];
            }
        }
        //排除两个单词前面prefix都一样，但是其中一个单词更加长
        return (n <= m);
    }
}
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