TrieIterator: Add iterator class

_trie_iterator.h

@@ -0,0 +1,24 @@
+#include <string>
+#include <iterator>
+
+#include "_trie_node.h"
+
+template <typename T>
+class trie_iterator {
+  private:
+    tnode<T>* root;
+  public:
+    using iterator_category = std::bidirectional_iterator_tag;
+    using difference_type = std::ptrdiff_t;
+    using value_type = std::pair<std::string, T>;
+    using pointer = value_type*;
+    using reference = value_type&;
+
+    // Constructors
+    explicit trie_iterator(tnode<T>*);
+};
+
+template <typename T>
+trie_iterator<T>::trie_iterator(tnode<T>* root) {
+  this->root = root;
+}

trie.h

@@ -1,10 +1,12 @@
 #include <string>
 
-#include "_trie_node.h"
+#include "_trie_iterator.h"
 
 template <typename T>
 class trie {
   public:
+    using iterator = trie_iterator<T>;
+
     trie();
     void insert(std::string, T);
     bool exist(std::string);