cs: add «1400. Construct K Palindrome Strings»

Count directly elements matching the parity check, do not prefilter and
count what's been included.

URL:	https://leetcode.com/problems/construct-k-palindrome-strings/
Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/construct-k-palindrome-strings.cs

@@ -11,6 +11,6 @@ public class Solution {
             parities[c - 'a'] ^= 1;
         }
 
-        return parities.Where(p => p != 0).Count() <= k;
+        return parities.Count(p => p != 0) <= k;
     }
 }