solve: Find the Duplicate Number

Author: 0x01f7

README.md

@@ -12,7 +12,7 @@
 | 290 | [Word Pattern][p290]                                                | [Ruby][s290]  | Easy        |
 | 289 | [Game of Life][p289]                                                | [Ruby][s289]  | Medium      |
 | 288 | [Unique Word Abbreviation][p288]                                    |    :lock:     | Easy        |
-| 287 | [Find the Duplicate Number][p287]                                   |               | Hard        |
+| 287 | [Find the Duplicate Number][p287]                                   | [Ruby][s287]  | Hard        |
 | 286 | [Walls and Gates][p286]                                             |    :lock:     | Medium      |
 | 285 | [Inorder Successor in BST][p285]                                    |    :lock:     | Medium      |
 | 284 | [Peeking Iterator][p284]                                            |    :soon:     | Medium      |
@@ -570,6 +570,7 @@
 [s292]:./algorithms/nim_game.rb
 [s290]:./algorithms/word_pattern.rb
 [s289]:./algorithms/game_of_life.rb
+[s287]:./algorithms/find_the_duplicate_number.rb
 [s283]:./algorithms/move_zeroes.rb
 [s282]:./algorithms/expression_add_operators.rb
 [s279]:./algorithms/perfect_squares.rb

algorithms/find_the_duplicate_number.rb

@@ -0,0 +1,37 @@
+# https://leetcode.com/problems/find-the-duplicate-number/
+#
+# Given an array nums containing n + 1 integers where each integer is
+# between 1 and n (inclusive), prove that at least one duplicate
+# number must exist. Assume that there is only one duplicate number,
+# find the duplicate one.
+#
+# Note:
+#
+#     + You must not modify the array (assume the array is read only).
+#     + You must use only constant, O(1) extra space.
+#     + Your runtime complexity should be less than O(n2).
+#     + There is only one duplicate number in the array, but it could
+#       be repeated more than once.
+
+
+# @param {Integer[]} nums
+# @return {Integer}
+def find_duplicate(nums)
+  start = nums.size
+
+  fast, slow = start, start
+  loop do
+    fast = nums[nums[fast - 1] - 1]
+    slow = nums[slow - 1]
+    break if fast == slow
+  end
+
+  slow = start
+  loop do
+    fast = nums[fast - 1]
+    slow = nums[slow - 1]
+    break if fast == slow
+  end
+
+  fast
+end