DaleStudy#291 Alien Dictionary

Author: forest000014

alien-dictionary/forest000014.java

@@ -0,0 +1,111 @@
+/*
+# Time Complexity: O(wl)
+  - w은 words의 길이, l은 words[i]의 길이
+  - word 하나를 trie에 등록하는 과정 O(l)
+  - 모든 word를 trie에 등록하는 과정 O(wl)
+  - graph를 순회하면서 위상정렬하는 과정 O(26^2) = O(1)
+# Space Complexity: O(wl)
+  - trie의 공간복잡도 O(wl)
+*/
+
+class Solution {
+
+    private class TrieNode {
+        char ch;
+        boolean ends;
+        List<TrieNode> children;
+
+        TrieNode(char ch) {
+            this.ch = ch;
+            children = new ArrayList<>();
+        }
+    }
+
+    public String alienOrder(String[] words) {
+        List<List<Integer>> graph = new ArrayList<>();
+        for (int i = 0; i < 26; i++) {
+            graph.add(new ArrayList<>());
+        }
+        boolean[] visited = new boolean[26];
+        int[] inDegree = new int[26];
+        int[] outDegree = new int[26];
+        Queue<Character> queue = new LinkedList<>();
+
+        TrieNode root = new TrieNode('.');
+
+        for (int i = 0; i < words.length; i++) {
+            TrieNode curr = root;
+
+            for (int j = 0; j < words[i].length(); j++) {
+                // 유효한 순서가 아님이 확실하면, 곧바로 false를 리턴한다.
+                // 유효한 순서가 아님이 확실하지 않으면, trie에 추가하고, relation을 추가한다.
+                // 단, words[i]의 마지막 글자라면, trie의 마지막에 ends = true를 세팅한다.
+
+                char ch = words[i].charAt(j);
+                visited[ch - 'a'] = true;
+
+                if (curr.children.size() == 0) {
+                    curr.children.add(new TrieNode(ch));
+                    curr = curr.children.get(curr.children.size() - 1);
+                    if (j == words[i].length()) curr.ends = true;
+                    continue;
+                }
+
+                char lastCh = curr.children.get(curr.children.size() - 1).ch;
+                if (lastCh == ch) {
+                    curr = curr.children.get(curr.children.size() - 1);
+                    if (j == words[i].length() - 1) {
+                        if (!curr.children.isEmpty()) return "";
+                        else curr.ends = true;
+                    }
+                    continue;
+                }
+
+                for (int p = 0; p < curr.children.size() - 1; p++) {
+                    if (curr.children.get(p).ch == ch) return "";
+                }
+
+                addEdge(graph, inDegree, outDegree, lastCh, ch);
+                curr.children.add(new TrieNode(ch));
+                curr = curr.children.get(curr.children.size() - 1);
+            }
+        }
+
+        for (int i = 0; i < 26; i++) {
+            if (inDegree[i] == 0 && outDegree[i] != 0) queue.offer((char)('a' + i));
+        }
+
+        StringBuilder sb = new StringBuilder();
+
+        for (int i = 0; i < 26; i++) {
+            if (visited[i] && inDegree[i] == 0 && outDegree[i] == 0) sb.append((char)('a' + i));
+        }
+
+        while (!queue.isEmpty()) {
+            char ch = queue.poll();
+            sb.append(ch);
+
+            for (int next : graph.get(ch - 'a')) {
+                if (--inDegree[next] == 0) queue.offer((char)('a' + next));
+            }
+        }
+
+        for (int i = 0; i < 26; i++) {
+            if (inDegree[i] > 0) return "";
+        }
+
+        return sb.toString();
+    }
+
+    private boolean addEdge(List<List<Integer>> graph, int[] inDegree, int[] outDegree, char from, char to) {
+        int sz = graph.get(from - 'a').size();
+        for (int i = 0; i < sz; i++) {
+            if (graph.get(from - 'a').get(i) == to - 'a') return false;
+        }
+
+        graph.get(from - 'a').add(to - 'a');
+        outDegree[from - 'a']++;
+        inDegree[to - 'a']++;
+        return true;
+    }
+}