Added 26 solutions

Data Structures/Trie/Contacts/Solution.java

@@ -0,0 +1,65 @@
+//     Author: Rodney Shaghoulian
+//     Github: github.com/rshaghoulian
+// HackerRank: hackerrank.com/rshaghoulian
+
+import java.util.Scanner;
+import java.util.HashMap;
+
+public class Solution {
+
+    private static TrieNode root = new TrieNode();
+    
+    public static void main(String[] args) {
+        Scanner scan = new Scanner(System.in);
+        int n = scan.nextInt();
+        for(int a0 = 0; a0 < n; a0++) {
+            String op = scan.next();
+            String contact = scan.next();
+            if (op.equals("add")) {
+                add(contact);
+            } else if (op.equals("find")) {
+                System.out.println(find(contact));
+            }
+        }
+        scan.close();
+    }
+    
+    private static void add(String str) {
+        TrieNode curr = root;
+        for (int i = 0; i < str.length(); i++) {
+            Character ch = str.charAt(i);
+            
+            if ( ! curr.children.containsKey(ch)) {
+                curr.addChild(ch);
+            }
+            curr = curr.children.get(ch);
+            curr.size++; // each time we visit a node (while adding letters), we increment this.
+        }
+    }
+    
+    private static int find(String str) {
+        TrieNode curr = root;
+        
+        /* Traverse down tree to end of our prefix */
+        for (int i = 0; i < str.length(); i++) {
+            Character ch = str.charAt(i);
+            if ( ! curr.children.containsKey(ch)) {
+                return 0;
+            } else {
+                curr = curr.children.get(ch);
+            }
+        }
+        
+        return curr.size;
+    }
+    
+    /* This implementation is somewhat based off the tutorial video in this problem */
+    public static class TrieNode {
+        public HashMap<Character, TrieNode> children = new HashMap<>();
+        public int size = 0; // this was the main trick to decrease runtime to pass tests.
+        
+        public void addChild(Character ch) {
+            children.put(ch, new TrieNode());
+        }
+    }
+}