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161
leetcode_java/src/main/java/LeetCodeJava/Graph/AlienDictionary.java
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,161 @@ | ||
| package LeetCodeJava.Graph; | ||
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| // https://leetcode.com/problems/alien-dictionary/description/ | ||
| // https://leetcode.ca/all/269.html | ||
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| import java.util.*; | ||
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| public class AlienDictionary { | ||
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| // V0 | ||
| // TODO : implement it | ||
| // public String alienOrder(String[] words){ | ||
| // return null; | ||
| // } | ||
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| // V1 | ||
| // IDEA : topological sorting | ||
| // https://leetcode.ca/all/269.html | ||
| // dfs | ||
| public String alienOrder_1(String[] words) { | ||
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| // Step 1: build the graph | ||
| Map<Character, Set<Character>> graph = new HashMap<>(); | ||
| for (int i = 0; i < words.length; i++) { | ||
| String currentWord = words[i]; | ||
| for (int j = 0; j < currentWord.length(); j++) { | ||
| if (!graph.containsKey(currentWord.charAt(j))) { | ||
| graph.put(currentWord.charAt(j), new HashSet<>()); | ||
| } | ||
| } | ||
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| if (i > 0) { | ||
| connectGraph(graph, words[i - 1], currentWord); | ||
| } | ||
| } | ||
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| // Step 2: topological sorting | ||
| StringBuffer sb = new StringBuffer(); | ||
| Map<Character, Integer> visited = new HashMap<Character, Integer>(); // mark as visited: visited.put(vertexId, -1); | ||
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| for (Map.Entry<Character, Set<Character>> entry: graph.entrySet()) { | ||
| char vertexId = entry.getKey(); | ||
| if (!topologicalSort(vertexId, graph, sb, visited)) { | ||
| return ""; | ||
| } | ||
| } | ||
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| return sb.toString(); | ||
| } | ||
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| private void connectGraph(Map<Character, Set<Character>> graph, String prev, String curr) { | ||
| if (prev == null || curr == null) { | ||
| return; | ||
| } | ||
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| int len = Math.min(prev.length(), curr.length()); | ||
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| for (int i = 0; i < len; i++) { | ||
| char p = prev.charAt(i); | ||
| char q = curr.charAt(i); | ||
| if (p != q) { // so if same duplicated work, will not reach here and not update graph | ||
| if (!graph.get(p).contains(q)) { | ||
| graph.get(p).add(q); | ||
| } | ||
| break; | ||
| } | ||
| } | ||
| } | ||
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| private boolean topologicalSort( | ||
| char vertexId, | ||
| Map<Character, Set<Character>> graph, | ||
| StringBuffer sb, | ||
| Map<Character, Integer> visited | ||
| ) { | ||
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| if (visited.containsKey(vertexId)) { | ||
| // visited | ||
| if (visited.get(vertexId) == -1) { // -1 meaning visited, cycle found | ||
| return false; | ||
| } | ||
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| // already in the list | ||
| if (visited.get(vertexId) == 1) { | ||
| return true; | ||
| } | ||
| } | ||
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| visited.put(vertexId, -1); // mark as visited | ||
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| Set<Character> neighbors = graph.get(vertexId); | ||
| for (char neighbor : neighbors) { | ||
| if (!topologicalSort(neighbor, graph, sb, visited)) { | ||
| return false; | ||
| } | ||
| } | ||
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| sb.insert(0, vertexId); | ||
| visited.put(vertexId, 1); // restore visited | ||
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| return true; | ||
| } | ||
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| // V2 | ||
| // https://github.com/Cee/Leetcode/blob/master/269%20-%20Alien%20Dictionary.java | ||
| public String alienOrder_2(String[] words) { | ||
| Map<Character, Set<Character>> map = new HashMap<>(); | ||
| Map<Character, Integer> degree = new HashMap<>(); | ||
| String result = ""; | ||
| if (words == null || words.length == 0) { return result; } | ||
| // Degree char = 0 | ||
| for (String s: words) { | ||
| for (char c: s.toCharArray()) { | ||
| degree.put(c, 0); | ||
| } | ||
| } | ||
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| for (int i = 0; i < words.length - 1; i++) { | ||
| String curr = words[i]; | ||
| String next = words[i + 1]; | ||
| int min = Math.min(curr.length(), next.length()); | ||
| for (int j = 0; j < min; j++) { | ||
| char c1 = curr.charAt(j); | ||
| char c2 = next.charAt(j); | ||
| if (c1 != c2) { | ||
| Set<Character> set = map.getOrDefault(c1, new HashSet<>()); | ||
| if (!set.contains(c2)) { | ||
| set.add(c2); | ||
| map.put(c1, set); | ||
| degree.put(c2, degree.get(c2) + 1); // update c2, c1 < c2 | ||
| } | ||
| break; | ||
| } | ||
| } | ||
| } | ||
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| LinkedList<Character> q = new LinkedList<>(); | ||
| for (char c: degree.keySet()) { | ||
| if (degree.get(c) == 0) { | ||
| q.add(c); | ||
| } | ||
| } | ||
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| while (!q.isEmpty()) { | ||
| char c = q.poll(); | ||
| result += c; | ||
| if (map.containsKey(c)) { | ||
| for (char next: map.get(c)) { | ||
| degree.put(next, degree.get(next) - 1); | ||
| if (degree.get(next) == 0) { | ||
| q.offer(next); | ||
| } | ||
| } | ||
| } | ||
| } | ||
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| return result.length() == degree.size() ? result : ""; | ||
| } | ||
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| } |