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anishLearnsToCodeanishLearnsToCode
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solves #269: Alien Dictionary in java
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‎README.md

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| 266 | 🔒 [Palindrome Permutation](https://leetcode.com/problems/palindrome-permutation) | | |
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| 267 | 🔒 [Palindrome Permutation II](https://leetcode.com/problems/palindrome-permutation-ii) | | |
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| 268 | [Missing Number](https://leetcode.com/problems/missing-number) | [![Java](assets/java.png)](src/MissingNumber.java) [![Python](assets/python.png)](python/missing_number.py) | |
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| 269 | [Alien Dictionary](https://leetcode.com/problems/alien-dictionary) | [![Java](assets/java.png)](src/AlienDictionary.java) | |
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| 270 | 🔒 [Closest Binary Search Tree Value](https://leetcode.com/problems/closest-binary-search-tree-value) | | |
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| 271 | 🔒 [Encode and Decode Strings](https://leetcode.com/problems/encode-and-decode-strings) | | |
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| 274 | [H-Index](https://leetcode.com/problems/h-index) | [![Java](assets/java.png)](src/HIndex.java) | |

‎src/AlienDictionary.java

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// https://leetcode.com/problems/alien-dictionary
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// T: O()
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// S: O()
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// C = length of all words added together
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// T: O(C)
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// S: O(1)
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import java.util.*;
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import java.util.ArrayList;
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import java.util.HashMap;
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import java.util.LinkedList;
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import java.util.List;
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import java.util.Map;
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import java.util.Queue;
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public class AlienDictionary {
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public String alienOrder(String[] words) {
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final Map<Character, Set<Character>> graph = createGraphFrom(words);
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return lexicographicallySmallest(graph);
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}
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private static String lexicographicallySmallest(Map<Character, Set<Character>> graph) {
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final Map<Character, Integer> inDegree = computeInDegree(graph);
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final Queue<Character> queue = new LinkedList<>();
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final StringBuilder builder = new StringBuilder();
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add0InDegreeInQueue(queue, inDegree);
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while (!queue.isEmpty()) {
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final char c = queue.poll();
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builder.append(c);
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for (char neighbour : graph.getOrDefault(c, new HashSet<>())) {
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inDegree.put(neighbour, inDegree.get(neighbour) - 1);
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if (inDegree.get(neighbour) == 0) {
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queue.add(neighbour);
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}
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// Step 0: Create data structures and find all unique letters.
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Map<Character, List<Character>> adjList = new HashMap<>();
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Map<Character, Integer> counts = new HashMap<>();
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for (String word : words) {
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for (char c : word.toCharArray()) {
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counts.put(c, 0);
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adjList.put(c, new ArrayList<>());
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}
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graph.remove(c);
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}
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if (graph.isEmpty()) {
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return builder.toString();
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}
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return "";
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}
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private static Map<Character, Set<Character>> createGraphFrom(String[] words) {
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final Map<Character, Set<Character>> graph = new HashMap<>();
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addAllLettersInGraph(graph, words);
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for (int i = 0 ; i < words.length ; i++) {
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for (int j = i + 1 ; j < words.length ; j++) {
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if (words[i].equals(words[j])) {
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continue;
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}
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final char[] charDiff = firstDifferentCharacter(words[i], words[j]);
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if (charDiff.length == 0) {
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continue;
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}
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final Set<Character> set = graph.getOrDefault(charDiff[0], new HashSet<>());
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set.add(charDiff[1]);
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graph.putIfAbsent(charDiff[0], set);
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// Step 1: Find all edges.
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for (int i = 0; i < words.length - 1; i++) {
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String word1 = words[i];
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String word2 = words[i + 1];
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// Check that word2 is not a prefix of word1.
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if (word1.length() > word2.length() && word1.startsWith(word2)) {
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return "";
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}
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}
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return graph;
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}
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private static void addAllLettersInGraph(Map<Character, Set<Character>> graph, String[] words) {
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for (String word : words) {
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for (int i = 0 ; i < word.length() ; i++) {
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graph.putIfAbsent(word.charAt(i), new HashSet<>());
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// Find the first non match and insert the corresponding relation.
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for (int j = 0; j < Math.min(word1.length(), word2.length()); j++) {
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if (word1.charAt(j) != word2.charAt(j)) {
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adjList.get(word1.charAt(j)).add(word2.charAt(j));
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counts.put(word2.charAt(j), counts.get(word2.charAt(j)) + 1);
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break;
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}
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}
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}
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}
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private static char[] firstDifferentCharacter(String s1, String s2) {
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for (int i = 0 ; i < s1.length() && i < s2.length() ; i++) {
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if (s1.charAt(i) != s2.charAt(i)) {
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return new char[] { s1.charAt(i), s2.charAt(i) };
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// Step 2: Breadth-first search.
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StringBuilder sb = new StringBuilder();
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Queue<Character> queue = new LinkedList<>();
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for (Character c : counts.keySet()) {
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if (counts.get(c).equals(0)) {
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queue.add(c);
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}
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}
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return new char[] {};
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}
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private static Map<Character, Integer> computeInDegree(Map<Character, Set<Character>> graph) {
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final Map<Character, Integer> inDegree = new HashMap<>();
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for (Set<Character> set : graph.values()) {
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for (char c : set) {
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inDegree.put(c, inDegree.getOrDefault(c, 0) + 1);
52+
while (!queue.isEmpty()) {
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Character c = queue.remove();
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sb.append(c);
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for (Character next : adjList.get(c)) {
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counts.put(next, counts.get(next) - 1);
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if (counts.get(next).equals(0)) {
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queue.add(next);
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}
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}
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}
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return inDegree;
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}
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private static void add0InDegreeInQueue(Queue<Character> queue, Map<Character, Integer> inDegree) {
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for (Map.Entry<Character, Integer> entry : inDegree.entrySet()) {
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if (entry.getValue() == 0) {
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queue.add(entry.getKey());
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}
63+
if (sb.length() < counts.size()) {
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return "";
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}
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return sb.toString();
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}
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}

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