Merge pull request keon#180 from SaadBenn/master

Add another problem

README.md

@@ -26,6 +26,7 @@ Thanks for your interest in contributing! There are many ways to contribute to t
     - [three_sum](array/three_sum.py)
     - [two_sum](array/two_sum.py)
     - [move_zeros_to_end](array/move_zeros_to_end.py)
+    - [delete_nth](array/delete_nth.py)
 - [backtrack](backtrack)
     - [general_solution.md](backtrack/)
     - [anagram](backtrack/anagram.py)

array/delete_nth.py

@@ -0,0 +1,42 @@
+"""
+Given a list lst and a number N, create a new list that contains each number of the list at most N times without reordering. 
+
+For example if N = 2, and the input is [1,2,3,1,2,1,2,3], you take [1,2,3,1,2], 
+drop the next [1,2] since this would lead to 1 and 2 being in the result 3 times, and then take 3, 
+which leads to [1,2,3,1,2,3]
+"""
+
+# Time complexity O(n^2)
+def delete_nth(order,max_e):
+    ans = []
+    for o in order:
+        if ans.count(o) < max_e: ans.append(o)
+    return ans
+
+# Another approach to solve the same problem using hash tables
+# Run time O(n) but O(1) look up time
+
+def delete_nth(order, max_e):
+  result = [] # add the final result into this list
+  my_dict = {} # keep track of occurences
+
+  for i in order:
+      if not i in my_dict:
+          # first time so add it to both the dict as well as the result list
+          my_dict[i] = 1  
+          result.append(i)
+      else:
+          my_dict[i] += 1 # increment the value
+          count = my_dict[i]
+          # we still need to add the element if the counter is less than the number specified
+          if count <= max_e:
+              result.append(i)
+  return result
+
+
+# Some test cases
+# Test.assert_equals(delete_nth([20,37,20,21], 1), [20,37,21], "From list [20,37,20,21],1 you get")
+# Test.assert_equals(delete_nth([1,1,3,3,7,2,2,2,2], 3), [1, 1, 3, 3, 7, 2, 2, 2], "From list [1,1,3,3,7,2,2,2,2],3 you get ")
+# Test.assert_equals(delete_nth([1, 2, 3, 1, 1, 2, 1, 2, 3, 3, 2, 4, 5, 3, 1],3), [1, 2, 3, 1, 1, 2, 2, 3, 3, 4, 5], "From list [1, 2, 3, 1, 1, 2, 1, 2, 3, 3, 2, 4, 5, 3, 1],3 you get ")
+# Test.assert_equals(delete_nth([1,1,1,1,1], 5), [1,1,1,1,1], "From list [1,1,1,1,1],5 you get ")
+# Test.assert_equals(delete_nth([], 5), [], "From list [],5 you get")