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AlienDictionary.java
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AlienDictionary.java
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package LeetCodeJava.Graph;
// https://leetcode.com/problems/alien-dictionary/description/
// https://leetcode.ca/all/269.html
import java.util.*;
public class AlienDictionary {
// V0
// TODO : implement it
// public String alienOrder(String[] words){
// return null;
// }
// V1
// IDEA : topological sorting
// https://leetcode.ca/all/269.html
// dfs
public String alienOrder_1(String[] words) {
// 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<>());
}
}
if (i > 0) {
connectGraph(graph, words[i - 1], currentWord);
}
}
// Step 2: topological sorting
StringBuffer sb = new StringBuffer();
Map<Character, Integer> visited = new HashMap<Character, Integer>(); // mark as visited: visited.put(vertexId, -1);
for (Map.Entry<Character, Set<Character>> entry: graph.entrySet()) {
char vertexId = entry.getKey();
if (!topologicalSort(vertexId, graph, sb, visited)) {
return "";
}
}
return sb.toString();
}
private void connectGraph(Map<Character, Set<Character>> graph, String prev, String curr) {
if (prev == null || curr == null) {
return;
}
int len = Math.min(prev.length(), curr.length());
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;
}
}
}
private boolean topologicalSort(
char vertexId,
Map<Character, Set<Character>> graph,
StringBuffer sb,
Map<Character, Integer> visited
) {
if (visited.containsKey(vertexId)) {
// visited
if (visited.get(vertexId) == -1) { // -1 meaning visited, cycle found
return false;
}
// already in the list
if (visited.get(vertexId) == 1) {
return true;
}
}
visited.put(vertexId, -1); // mark as visited
Set<Character> neighbors = graph.get(vertexId);
for (char neighbor : neighbors) {
if (!topologicalSort(neighbor, graph, sb, visited)) {
return false;
}
}
sb.insert(0, vertexId);
visited.put(vertexId, 1); // restore visited
return true;
}
// 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);
}
}
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;
}
}
}
LinkedList<Character> q = new LinkedList<>();
for (char c: degree.keySet()) {
if (degree.get(c) == 0) {
q.add(c);
}
}
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);
}
}
}
}
return result.length() == degree.size() ? result : "";
}
}