alien dictionary solution

Author: yhkee0404

alien-dictionary/yhkee0404.kt

@@ -0,0 +1,63 @@
+class Solution {
+    /**
+     * @param words: a list of words
+     * @return: a string which is correct order
+     */
+    fun alienOrder(words: Array<String>): String {
+        // Write your code here
+        val adj = List<MutableList<Int>>('z'.code - 'a'.code + 1) {mutableListOf()}
+        for (i in 1 until words.size) {
+            var j = 0
+            while (j != words[i - 1].length && j != words[i].length && words[i - 1][j] == words[i][j]) {
+                j++
+            }
+            if (j == words[i - 1].length) {
+                continue
+            }
+            if (j == words[i].length) {
+                return ""
+            }
+            adj[words[i - 1][j].code - 'a'.code].add(words[i][j].code - 'a'.code)
+        }
+        val visited = MutableList(adj.size) {false}
+        words.forEach {
+                    it.map { it.code - 'a'.code }
+                            .forEach { visited[it] = true }
+                }
+        val inDegrees = MutableList(adj.size) {0} // Kahn T(V, E) = S(V, E) = O(V + E)
+        adj.forEach {
+                    it.forEach { inDegrees[it]++ }
+                }
+        val ans = mutableListOf<Int>()
+        val stack = mutableListOf<Int>()
+        (0 until adj.size).filter { visited[it] && inDegrees[it] == 0 }
+                .forEach {
+                    stack.add(it)
+                    ans.add(it)
+                }
+        while (! stack.isEmpty()) {
+            val u = stack.removeLast()
+            adj[u].forEach {
+                        if (--inDegrees[it] == 0) {
+                            stack.add(it)
+                            ans.add(it)
+                        }
+                    }
+        }
+        if ((0 until adj.size).any { visited[it] && inDegrees[it] != 0 }) {
+            return ""
+        }
+        return ans.map { (it + 'a'.code).toChar() }
+                .joinToString("")
+        /*val decoder = MutableList(adj.size) {0}
+        (0 until ans.size).forEach { decoder[ans[it]] = it }
+        val decoded = words.map {
+                    it.map { (decoder[it.code - 'a'.code] + 'a'.code).toChar() }
+                            .joinToString("")
+                }
+        val sortedWords = decoded.sorted()
+        return if (decoded == sortedWords) ans.map { (it + 'a'.code).toChar() }
+                        .joinToString("")
+            else ""*/
+    }
+}