solves trie #208 in java

Author: anishLearnsToCode

Diff for: README.md

@@ -168,7 +168,7 @@
 | 205  | [Isomorphic Strings](https://leetcode.com/problems/isomorphic-strings)                                                                                    |                                 [![Java](assets/java.png)](src/IsomorphicStrings.java) [![Python](assets/python.png)](python/isomorphic_strings.py)                                  |                                                                                                                                   |
 | 206  | [Reverse Linked List](https://leetcode.com/problems/reverse-linked-list)                                                                                  |                                 [![Java](assets/java.png)](src/ReverseLinkedList.java) [![Python](assets/python.png)](python/reverse_linked_list.py)                                 |                                                                                                                                   |
 | 207  | [Course Schedule](https://leetcode.com/problems/course-schedule)                                                                                          |                                                                 [![Java](assets/java.png)](src/CourseSchedule.java)                                                                  |                                                                                                                                   |
-| 208  | [Implement Trie (Prefix Tree)](https://leetcode.com/problems/implement-trie-prefix-tree)                                                                  |                                                                                                                                                                                      |                                                                                                                                   |
+| 208  | [Implement Trie (Prefix Tree)](https://leetcode.com/problems/implement-trie-prefix-tree)                                                                  |                                                                      [![Java](assets/java.png)](src/Trie.java)                                                                       |                                                                                                                                   |
 | 209  | [Minimum Size Subarray Sum](https://leetcode.com/problems/minimum-size-subarray-sum)                                                                      |                                                                                                                                                                                      |                                                                                                                                   |
 | 210  | [Course Schedule II](https://leetcode.com/problems/course-schedule-ii)                                                                                    |                                                                                                                                                                                      |                                                                                                                                   |
 | 211  | [Design Add and Search Words Data Structure](https://leetcode.com/problems/design-add-and-search-words-data-structure)                                    |                                                                                                                                                                                      |                                                                                                                                   |

Diff for: src/HelloWorld.java

@@ -1,5 +1,11 @@
 public class HelloWorld {
     public static void main(String[] args) {
-        System.out.println("Hello world");
+        Trie trie = new Trie();
+        trie.insert("apple");
+//        System.out.println(trie.search("apple"));
+//        System.out.println(trie.search("app"));
+        System.out.println(trie.startsWith("app")); // true
+        trie.insert("app");
+//        System.out.println(trie.search("app"));
     }
 }

Diff for: src/Trie.java

@@ -0,0 +1,59 @@
+// https://leetcode.com/problems/implement-trie-prefix-tree
+// insertion
+// T: O(n)
+// S: O(n)
+// search
+// T: O(n)
+// S: O(n)
+// starts with
+// T: O(n)
+// S: O(n)
+
+public class Trie {
+    private final Trie[] alphabet = new Trie[26];
+    private boolean isWordEnd = false;
+
+    public Trie() { }
+
+    public void insert(String word) {
+        insert(word, 0);
+    }
+
+    private void insert(String word, int index) {
+        if (index == word.length()) {
+            this.isWordEnd = true;
+            return;
+        }
+        int charIndex = toIndex(word.charAt(index));
+        if (alphabet[charIndex] == null) {
+            alphabet[charIndex] = new Trie();
+        }
+        alphabet[charIndex].insert(word, index + 1);
+    }
+
+    public boolean search(String word) {
+        return search(word, 0);
+    }
+
+    public boolean search(String word, int index) {
+        if (index == word.length()) return isWordEnd;
+        int charIndex = toIndex(word.charAt(index));
+        if (alphabet[charIndex] == null) return false;
+        return alphabet[charIndex].search(word, index + 1);
+    }
+
+    public boolean startsWith(String prefix) {
+        return startsWith(prefix, 0);
+    }
+
+    public boolean startsWith(String prefix, int index) {
+        if (index == prefix.length()) return true;
+        int charIndex = toIndex(prefix.charAt(index));
+        if (alphabet[charIndex] == null) return false;
+        return alphabet[charIndex].startsWith(prefix, index + 1);
+    }
+
+    private int toIndex(char c) {
+        return c - 'a';
+    }
+}