chore(CSharp): add trie implementation (#613)

algorithms/CSharp/README.md

@@ -21,6 +21,7 @@ For running the `.cs` file please using [.Net Finddle](https://dotnetfiddle.net/
 
 ## Strings
 - [Palindrome](src/Strings/palindrome.cs)
+- [Trie](src/Strings/trie.cs)
 
 ## Search
 - [Binary Search](src/Search/binary-search.cs)

algorithms/CSharp/src/Strings/trie.cs

@@ -0,0 +1,119 @@
+using System;
+using System.Collections.Generic;
+
+namespace Algorithms.Strings
+{
+    public class Trie
+    {
+        const int ASCII_QUANTITY = 256;
+
+        private class TrieNode
+        {
+            public bool end;
+            public TrieNode[] next;
+
+            public TrieNode()
+            {
+                end = false;
+                next = new TrieNode[ASCII_QUANTITY];
+            }
+        }
+
+        private TrieNode _root;
+
+        public Trie()
+        {
+            _root = new TrieNode();
+        }
+
+        public void InsertWord(string word)
+        {
+            TrieNode current = _root;
+            for(int i = 0; i < word.Length; i++)
+            {
+                int id = word[i];
+                if(current.next[id] == null)
+                {
+                    current.next[id] = new TrieNode();
+                }
+
+                current = current.next[id];
+            }
+
+            current.end = true;
+        }
+
+        public bool SearchWord(string word)
+        {
+            TrieNode current = _root;
+            for(int i = 0; i < word.Length; i++)
+            {
+                int id = word[i];
+                if(current.next[id] == null)
+                {
+                    return false;
+                }
+
+                current = current.next[id];
+            }
+
+            return current.end;
+        }
+    }
+
+    public class TrieTester
+    { 
+        public static List<bool> TestTrie(List<Tuple<string, string>> operations)
+        {
+            List<bool> answer = new List<bool>();
+            Trie dictionary = new Trie();
+
+            foreach(var op in operations)
+            {
+                if(op.Item1 == "insert")
+                {
+                    dictionary.InsertWord(op.Item2);
+                }
+                else if(op.Item1 == "search")
+                {
+                    answer.Add(dictionary.SearchWord(op.Item2));
+                }
+            }
+
+            return answer;
+        }
+
+        public static void Main()
+        {
+            List<Tuple<string, string>> operations = new List<Tuple<string, string>>()
+            {
+                new Tuple<string, string>("insert", "allure"),
+                new Tuple<string, string>("search", "allure"),
+                new Tuple<string, string>("search", "cow"),
+                new Tuple<string, string>("search", "all"),
+                new Tuple<string, string>("insert", "all"),
+                new Tuple<string, string>("search", "all")
+            };
+
+            List<bool> results = TestTrie(operations);
+            Console.WriteLine(string.Join(", ", results));
+        }
+    }
+
+}
+
+/*
+ * Trie is an efficient data-structure to store words. This can be used for solving multiple problems, even in number
+ * theory!!
+ * Here's how it works,
+ * We keep a root node. Each path from root to a leaf is a word. Even a word can end in the middle of the path.
+ * For example, "allure" and "all". "all" is in "allure", so we will not need to insert it.
+ * The path defines a letter in the node from one node to another. In that case we can consider the root to be empty string.
+ * Each node in this implementation contains an array of size 256, for the ASCII codes and a boolean to mark end.
+ * From the root, it checks whether the current letter exists.
+ * If yes then it will just move to that path. Else it will create a new node and move.
+ * When inserting the first word, let "allure", new nodes will be created. the node for the last letter will be marked as end.
+ * When inserting "all", "all" already exists as prefix of "allure", so we will just mark the end variable of second 'l' as true.
+ * Search is like this too. We start from the root and the first character of the word. If a character is found, we move forward else return false.
+ * Time complexity of Search is O(length). The insert operation takes O(length) time.
+ */

algorithms/CSharp/test/Strings/trie.cs

@@ -0,0 +1,33 @@
+using System;
+using NUnit.Framework;
+using System.Collections.Generic;
+
+namespace Algorithms.Tests.Strings
+{
+    public class Trie
+    {
+        static object[] TestCasesForTrie =
+{
+            new object[] 
+            {
+                new List<Tuple<string, string>>()
+                {
+                    new Tuple<string, string>("insert", "allure"),
+                    new Tuple<string, string>("search", "allure"),
+                    new Tuple<string, string>("search", "cow"),
+                    new Tuple<string, string>("search", "all"),
+                    new Tuple<string, string>("insert", "all"),
+                    new Tuple<string, string>("search", "all")
+                },
+                new List<bool>() { true, false, false, true}
+            },
+        };
+
+        [TestCaseSource(nameof(TestCasesForTrie))]
+        public void TestTrie_ShouldGetExpectedResult(List<Tuple<string, string>> operations, List<bool> expected)
+        {
+            List<bool> result = Algorithms.Strings.TrieTester.TestTrie(operations);
+            Assert.AreEqual(expected, result);
+        }
+    }
+}