better solution of Find The Duplicate Number

Author: P-ppc

algorithms/FindTheDuplicateNumber/README.md

@@ -1,2 +1,34 @@
 # Find The Duplicate Number
-We can solve this problem by Binary Search
+We can solve this problem by Binary Search. The better solution is using double pointer like below:
+```python
+class Solution(object):
+    def findDuplicate(self, nums):
+        """
+        :type nums: List[int]
+        :rtype: int
+        """
+        # The "tortoise and hare" step.  We start at the end of the array and try
+        # to find an intersection point in the cycle.
+        slow = 0
+        fast = 0
+    
+        # Keep advancing 'slow' by one step and 'fast' by two steps until they
+        # meet inside the loop.
+        while True:
+            slow = nums[slow]
+            fast = nums[nums[fast]]
+    
+            if slow == fast:
+                break
+    
+        # Start up another pointer from the end of the array and march it forward
+        # until it hits the pointer inside the array.
+        finder = 0
+        while True:
+            slow   = nums[slow]
+            finder = nums[finder]
+    
+            # If the two hit, the intersection index is the duplicate element.
+            if slow == finder:
+                return slow
+```