solves find the duplicate element #287 in java

Author: anishLearnsToCode

README.md

@@ -234,7 +234,7 @@
 | 284  | [Peeking Iterator](https://leetcode.com/problems/peeking-iterator)                                                                                        |                                                                                       [![Java](assets/java.png)](src/PeekingIterator.java)                                                                                       |                                                                                                                                   |
 | 285  | 🔒 [Inorder Successor in BST](https://leetcode.com/problems/inorder-successor-in-bst)                                                                     |                                                                                                                                                                                                                                  |                                                                                                                                   |
 | 286  | 🔒 [Walls and Gates](https://leetcode.com/problems/walls-and-gates)                                                                                       |                                                                                                                                                                                                                                  |                                                                                                                                   |
-| 287  | [Find the Duplicate Number](https://leetcode.com/problems/find-the-duplicate-number)                                                                      |                                                                                                                                                                                                                                  |                                                                                                                                   |
+| 287  | [Find the Duplicate Number](https://leetcode.com/problems/find-the-duplicate-number)                                                                      |                                                                                   [![Java](assets/java.png)](src/FindTheDuplicateNumber.java)                                                                                    |                                                                                                                                   |
 | 288  | 🔒 [Unique Word Abbreviation](https://leetcode.com/problems/unique-word-abbreviation)                                                                     |                                                                                                                                                                                                                                  |                                                                                                                                   |
 | 289  | [Game of Life](https://leetcode.com/problems/game-of-life)                                                                                                |                                                                                                                                                                                                                                  |                                                                                                                                   |
 | 290  | [Word Pattern](https://leetcode.com/problems/word-pattern)                                                                                                |                                                             [![Java](assets/java.png)](src/WordPattern.java) [![Python](assets/python.png)](python/word_pattern.py)                                                              |                                                                                                                                   |

src/FindTheDuplicateNumber.java

@@ -0,0 +1,26 @@
+// https://leetcode.com/problems/find-the-duplicate-number
+// T: O(N)
+// S: O(1)
+
+public class FindTheDuplicateNumber {
+    public static int findDuplicate(int[] nums) {
+        int tortoise = nums[0];
+        int hare = nums[0];
+
+        do {
+            tortoise = nums[tortoise];
+            hare = nums[nums[hare]];
+        } while (tortoise != hare);
+
+        // Find the "entrance" to the cycle.
+        tortoise = nums[0];
+
+        while (tortoise != hare) {
+            tortoise = nums[tortoise];
+            hare = nums[hare];
+        }
+
+        return hare;
+    }
+}
+