Merge pull request #179 from Tr-Jono/master

Add trie implementation

Author: ionicc

Data_Structure/src/Trees/trie.py

@@ -0,0 +1,129 @@
+"""
+Trie implementation in Python by @Tr-Jono
+
+A trie is a type of search tree that is used for storing strings.
+Visual representation of a trie with strings "ab", "ac", "ace", "ba", "boom":
+
+      [root]
+      /    \
+   "a"     "b"
+   / \     / \
+ "b" "c" "a" "o"
+      |       |
+     "e"     "o"
+              |
+             "m"
+
+The time complexities of addition, removal and querying existence/count of a string are all O(|target string|).
+
+A trie could be used in place of a string list used for searching.
+Time complexity of searching a string in:
+    - a list: O(|list|)
+    - a trie: O(|target string|)
+
+This implementation of the trie relies on recursively stored _TrieNode instances.
+The Trie class, acting as the root, implements typical data structure methods.
+"""
+
+
+class _TrieNode:
+    def __init__(self):
+        self.words = 0
+        self.children = {}
+
+    def __len__(self):
+        return self.words + sum(len(c) for c in self.children.values())
+
+    def add(self, s):
+        if not s:
+            self.words += 1
+            return
+        if s[0] not in self.children:
+            self.children[s[0]] = _TrieNode()
+        self.children[s[0]].add(s[1:])
+
+    def remove(self, s):
+        if not s:
+            if not self.words:
+                raise ValueError
+            self.words -= 1
+            return
+        if s[0] not in self.children:
+            raise ValueError
+        self.children[s[0]].remove(s[1:])
+
+    def count(self, s):
+        if not s:
+            return self.words
+        if s[0] not in self.children:
+            return 0
+        return self.children[s[0]].count(s[1:])
+
+    def get_words(self):
+        words = [""] * self.words
+        for c in self.children:
+            for w in self.children[c].get_words():
+                words.append(c + w)
+        return words
+
+
+class Trie:
+    def __init__(self, *words):
+        self.children = {}
+        for w in words:
+            self.add(w)
+
+    def __contains__(self, item):
+        return bool(self.count(item))
+
+    def __len__(self):
+        return sum(len(c) for c in self.children.values())
+
+    def __bool__(self):
+        return bool(len(self))
+
+    def __repr__(self):
+        return "{}({})".format(self.__class__.__name__, ", ".join([repr(w) for w in self.get_words()]))
+
+    def add(self, s):
+        if not s:
+            raise ValueError("Empty strings disallowed")
+        if s[0] not in self.children:
+            self.children[s[0]] = _TrieNode()
+        self.children[s[0]].add(s[1:])
+
+    def remove(self, s):
+        if not s:
+            raise ValueError("Empty strings disallowed")
+        try:
+            if s[0] not in self.children:
+                raise ValueError
+            self.children[s[0]].remove(s[1:])
+        except ValueError:
+            raise ValueError("{} not in trie".format(repr(s))) from None
+
+    def count(self, s):
+        if s[0] not in self.children:
+            return 0
+        return self.children[s[0]].count(s[1:])
+
+    def get_words(self):
+        words = []
+        for c in self.children:
+            for w in self.children[c].get_words():
+                words.append(c + w)
+        return words
+
+
+def main():
+    trie = Trie("dog", "dog", "cat", "tiger")
+    trie.add("bird")
+    trie.remove("bird")
+    print("repr(trie):", trie)
+    print("len(trie):", len(trie))
+    print("trie.count('dog'):", trie.count("dog"))
+    print("'bird' in trie:", "bird" in trie)
+
+
+if __name__ == "__main__":
+    main()