diff --git a/0136_single-number/README.md b/0136_single-number/README.md
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+Given a **non-empty** array of integers `nums`, every element appears _twice_ except for one. Find that single one.
+
+You must implement a solution with a linear runtime complexity and use only constant extra space.
+
+**Example 1:**
+
+    Input: nums = [2,2,1]
+    Output: 1
+    
+
+**Example 2:**
+
+    Input: nums = [4,1,2,1,2]
+    Output: 4
+    
+
+**Example 3:**
+
+    Input: nums = [1]
+    Output: 1
+    
+
+**Constraints:**
+
+*   `1 <= nums.length <= 3 * 104`
+*   `-3 * 104 <= nums[i] <= 3 * 104`
+*   Each element in the array appears twice except for one element which appears only once.
+
+https://leetcode.com/problems/single-number/
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diff --git a/0136_single-number/python3/solution.py b/0136_single-number/python3/solution.py
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+from functools import reduce
+
+class Solution:
+    def singleNumber(self, nums: List[int]) -> int:
+        return reduce(lambda r, a: r ^ a, nums)