Add trie challenge

graphs_trees/trie/trie.py

@@ -0,0 +1,76 @@
+from collections import OrderedDict
+
+
+class Node(object):
+
+    def __init__(self, key, parent=None, terminates=False):
+        self.key = key
+        self.terminates = False
+        self.parent = parent
+        self.children = {}
+
+
+class Trie(object):
+
+    def __init__(self):
+        self.root = Node('')
+
+    def find(self, word):
+        if word is None:
+            raise TypeError('word cannot be None')
+        node = self.root
+        for char in word:
+            if char in node.children:
+                node = node.children[char]
+            else:
+                return None
+        return node if node.terminates else None
+
+    def insert(self, word):
+        if word is None:
+            raise TypeError('word cannot be None')
+        node = self.root
+        parent = None
+        for char in word:
+            if char in node.children:
+                node = node.children[char]
+            else:
+                node.children[char] = Node(char, parent=node)
+                node = node.children[char]
+        node.terminates = True
+
+    def remove(self, word):
+        if word is None:
+            raise TypeError('word cannot be None')
+        node = self.find(word)
+        if node is None:
+            raise KeyError('word does not exist')
+        node.terminates = False
+        parent = node.parent
+        while parent is not None:
+            # As we are propagating the delete up the 
+            # parents, if this node has children, stop
+            # here to prevent orphaning its children.
+            # Or
+            # if this node is a terminating node that is
+            # not the terminating node of the input word, 
+            # stop to prevent removing the associated word.
+            if node.children or node.terminates:
+                return
+            del parent.children[node.key]
+            node = parent
+            parent = parent.parent
+
+    def list_words(self):
+        result = []
+        curr_word = ''
+        self._list_words(self.root, curr_word, result)
+        return result
+
+    def _list_words(self, node, curr_word, result):
+        if node is None:
+            return
+        for key, child in node.children.items():
+            if child.terminates:
+                result.append(curr_word+key)
+            self._list_words(child, curr_word+key, result)