diff --git a/lib/exercises.rb b/lib/exercises.rb
index e1b3850..c9d5d59 100644
--- a/lib/exercises.rb
+++ b/lib/exercises.rb
@@ -1,29 +1,96 @@
 
 # This method will return an array of arrays.
 # Each subarray will have strings which are anagrams of each other
-# Time Complexity: ?
-# Space Complexity: ?
+# Time Complexity: O(n) * O(mlogm)
+# Space Complexity: O(n+m) = O(n)
 
 def grouped_anagrams(strings)
-  raise NotImplementedError, "Method hasn't been implemented yet!"
+
+  hash_table = Hash.new([])
+  strings.each do |word|                                      # time: O(n)
+    hash_table[word.split("").sort.join("")] += [word]        # time: O(mlogm), space: o(m) for split, o(n) for hash
+  end
+
+  return hash_table.values
 end
 
 # This method will return the k most common elements
 # in the case of a tie it will select the first occuring element.
-# Time Complexity: ?
-# Space Complexity: ?
+# Time Complexity: O(nlogn)
+# Space Complexity: O(n)
 def top_k_frequent_elements(list, k)
-  raise NotImplementedError, "Method hasn't been implemented yet!"
-end
 
+  hash_table = Hash.new(0)
+
+  list.each do |int|                            # time: o(n)
+    hash_table[int] += 1                        # space: o(n)
+  end
+
+  # sort by the values
+  sorted = hash_table.sort_by{|k, v| -v}         # time: o(nlogn), space: o(n)
+
+  # return top k
+  top_k = []                                     # space: o(m)
+  sorted[0..k-1].each do |pair|                  # time: o(m), space: o(m) for []
+    top_k.push(pair[0])                          # time: o(1)
+  end
+
+  return top_k.sort                              # time: o(mlogm)
+end
 
 # This method will return the true if the table is still
 #   a valid sudoku table.
 #   Each element can either be a ".", or a digit 1-9
 #   The same digit cannot appear twice or more in the same 
 #   row, column or 3x3 subgrid
-# Time Complexity: ?
-# Space Complexity: ?
+# Time Complexity: O(n^2) - where n and n are rows and cols
+# Space Complexity: O(n) - bc hash table created for every row/col separately
 def valid_sudoku(table)
-  raise NotImplementedError, "Method hasn't been implemented yet!"
+  # check cols
+  9.times do |col|
+    hash_table = {}
+    table.each_index do |row|
+      val = table[row][col]
+      if val != "."
+        return false if hash_table[val]
+        hash_table[val] = true
+      end
+    end
+  end
+
+  # check rows
+  table.each_index do |row|
+    hash_table = {}
+    table[row].each_index do |col|
+      val = table[row][col]
+      if val != "."
+        return false if hash_table[val]
+        hash_table[val] = true
+      end
+    end
+  end
+
+  # check sub-matrices
+  i = j = 0
+  until i == 9 || j == 9
+    return false unless matrix_helper(table, i, j)
+    i += 3
+    j += 3
+  end
+
+  return true
 end
+
+def matrix_helper(matrix, i, j)
+  hash_table = {}
+  3.times do |row|
+    3.times do|col|
+      val = matrix[row + i][col + j]
+      if val != "."
+        return false if hash_table[val]
+        hash_table[val] = true
+      end
+    end
+  end
+  return true
+end
\ No newline at end of file
diff --git a/test/exercises_test.rb b/test/exercises_test.rb
index 74646dc..8025075 100644
--- a/test/exercises_test.rb
+++ b/test/exercises_test.rb
@@ -151,7 +151,7 @@
     end
   end
 
-  xdescribe "valid sudoku" do
+  describe "valid sudoku" do
     it "works for the table given in the README" do
       # Arrange
       table = [