update

Author: kamyu104

README.md

@@ -231,7 +231,7 @@ Problem         |  Solution       |  Time           | Space           | Difficul
 [Max Points on a Line] | [max-points-on-a-line.py] | _O(n^2)_ | _O(n)_ | Hard          |
 [Minimum Window Substring] | [minimum-window-substring.py] | _O(n^2)_ | _O(n)_ | Hard          |
 [Substring with Concatenation of All Words] | [substring-with-concatenation-of-all-words.py] | _O(m * n * k)_ | _O(n * k)_ | Hard          |
-[Two Sum]      | [two-sum.py]      | _O(n)_         | _O(n)_          | Hard           |
+[Two Sum]      | [two-sum.py]      | _O(n)_         | _O(n)_          | Medium         |
 
 [Longest Substring Without Repeating Characters]:https://oj.leetcode.com/problems/longest-substring-without-repeating-characters/
 [longest-substring-without-repeating-characters.py]:https://github.com/kamyu104/LeetCode/blob/master/Python/longest-substring-without-repeating-characters.py