content: add new matrix question (#227)

This is an interesting question because it requires to break the matrix into a smaller matrix

contents/algorithms/matrix.md

@@ -34,3 +34,4 @@ transposed_matrix = zip(*matrix)
 - [Spiral Matrix](https://leetcode.com/problems/spiral-matrix/)
 - [Rotate Image](https://leetcode.com/problems/rotate-image/)
 - [Word Search](https://leetcode.com/problems/word-search/)
+- [Valid Sudoku](https://leetcode.com/problems/valid-sudoku/)