Add implement trie (prefix tree) solution

Author: hyunjung-choi

implement-trie-prefix-tree/hyunjung-choi.kt

@@ -0,0 +1,45 @@
+/**
+ * 시간복잡도: O(L) (insert, search, startsWith 모두 동일)
+ * 공간복잡도: O(ΣL × alphabetSize)
+ */
+
+class Trie() {
+
+    private class Node {
+        val child = arrayOfNulls<Node>(26)
+        var isEnd: Boolean = false
+    }
+
+    private val root = Node()
+
+    fun insert(word: String) {
+        var cur = root
+        for (ch in word) {
+            val i = ch - 'a'
+            if (cur.child[i] == null) {
+                cur.child[i] = Node()
+            }
+            cur = cur.child[i]!!
+        }
+        cur.isEnd = true
+    }
+
+    fun search(word: String): Boolean {
+        val node = findNode(word)
+        return node?.isEnd == true
+    }
+
+    fun startsWith(prefix: String): Boolean {
+        return findNode(prefix) != null
+    }
+
+    private fun findNode(s: String): Node? {
+        var cur = root
+        for (ch in s) {
+            val i = ch - 'a'
+            val next = cur.child[i] ?: return null
+            cur = next
+        }
+        return cur
+    }
+}