READMEs updated with links to leetcode

Author: Deftly

leetcode/0036.valid_sudoku/README.md

@@ -1,4 +1,4 @@
-# 36. Valid Sudoku
+# [36. Valid Sudoku](https://leetcode.com/problems/valid-sudoku/description/)
 Determine if a `9 x 9` Sudoku board is valid. Only the filled cells need to be validated according the following rules:
 - Each row must contain the digits `1-9` without repetition.
 - Each column must contain the digits `1-9` without repetition.

leetcode/0238.product_of_array_except_self/README.md

@@ -1,4 +1,4 @@
-# 238. Product of Array Except Self
+# [238. Product of Array Except Self](https://leetcode.com/problems/product-of-array-except-self/description/)
 Given an integer array `nums`, return an array `answer` such that `answer[i]` is equal to the product of all the elements of `nums` except `nums[i]`.
 
 The product of any prefix or suffix of `nums` is guaranteed to fit in a 32-bit integer.