Added medium programming challenge with detailed problem statement, e…

…xamples, and constraints.

programming_interview/medium_challenge.md

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+# Medium Programming Challenge
+
+## Problem Statement
+Write a function `find_longest_subarray` that takes a list of integers and returns the length of the longest contiguous subarray that contains only unique elements.
+
+## Input/Output Specifications
+- Input:
+  - A list of integers `nums` (1 <= len(nums) <= 10^5) where -10^4 <= nums[i] <= 10^4.
+- Output:
+  - An integer representing the length of the longest contiguous subarray with unique elements.
+
+## Examples
+- Input: [1, 2, 3, 4, 5]
+  Output: 5
+
+- Input: [1, 2, 2, 3, 4]
+  Output: 3
+
+- Input: [5, 5, 5, 5, 5]
+  Output: 1
+
+## Constraints
+- The list will have at least one integer.
+- The solution should have a time complexity of O(n), where n is the length of the input list.