implement trie prefix tree solution

Author: yhkee0404

implement-trie-prefix-tree/yhkee0404.dart

@@ -0,0 +1,40 @@
+class Trie {
+
+  final int n;
+  final children = <int, Trie>{};
+  bool ended = false;
+
+  Trie([this.n = 0]);
+  
+  void insert(String word) {
+    Trie u = this;
+    for (int rune in word.runes) {
+        u = u.children
+                .putIfAbsent(rune, () => Trie(rune));
+    }
+    u.ended = true;
+  }
+  
+  bool search(String word) => _search(word)?.ended ?? false;
+  
+  bool startsWith(String prefix) => _search(prefix) != null;
+
+  Trie? _search(String word) {
+    Trie? u = this;
+    for (int rune in word.runes) {
+        u = u!.children[rune];
+        if (u == null) {
+            return null;
+        }
+    }
+    return u;
+  }
+}
+
+/**
+ * Your Trie object will be instantiated and called as such:
+ * Trie obj = Trie();
+ * obj.insert(word);
+ * bool param2 = obj.search(word);
+ * bool param3 = obj.startsWith(prefix);
+ */