269. Alien Dictionary
本质上就是topological sort.
1. 统计所有的点
对于每一个string,把所有的字母都加入indegree
2. 构建图,统计indegree
对于没连续的一组str,找到第一个不同的字母,把后一个加入前一个字母的neighbor,并且给后一个字母indegree+1.
需要注意的是,如果要检查后一个字母是不是已经在第一个字母的neighbor,如果不是再做后续操作
3. 遍历图
注意可能没有邻节点
4. 检查是不是所有点都访问到了,返回结果
1 public String alienOrder(String[] words) { 2 if(words.length < 1) { 3 return ""; 4 } 5 String res = ""; 6 Map<Character, Set<Character>> map = new HashMap<Character, Set<Character>>(); 7 Map<Character, Integer> indegree = new HashMap<Character, Integer>(); 8 for(String s: words) { 9 for(char c: s.toCharArray()) { 10 indegree.put(c, 0); 11 } 12 } 13 for(int i = 0; i < words.length - 1; i++) { 14 String cur = words[i]; 15 String next = words[i + 1]; 16 int len = Math.min(cur.length(), next.length()); 17 for(int j = 0; j < len; j++) { 18 char c1 = cur.charAt(j); 19 char c2 = next.charAt(j); 20 if(c1 != c2) { 21 Set<Character> neighbors = map.get(c1); 22 if(neighbors == null) { 23 neighbors = new HashSet<Character>(); 24 } 25 if(!neighbors.contains(c2)) { 26 neighbors.add(c2); 27 map.put(c1, neighbors); 28 indegree.put(c2, indegree.get(c2) + 1); 29 } 30 break; 31 } 32 } 33 } 34 Queue<Character> queue = new LinkedList<Character>(); 35 for(Character c: indegree.keySet()) { 36 if(indegree.get(c) == 0) { 37 queue.offer(c); 38 } 39 } 40 while(!queue.isEmpty()) { 41 Character c = queue.poll(); 42 res += c; 43 Set<Character> neighbors = map.get(c); 44 if(neighbors == null) { 45 continue; 46 } 47 for(Character x: neighbors) { 48 indegree.put(x, indegree.get(x) - 1); 49 if(indegree.get(x) == 0) { 50 queue.offer(x); 51 } 52 } 53 } 54 return indegree.size() == res.length()? res: ""; 55 }