cs: add «2657. Find the Prefix Common Array of Two Arrays»

Use ‹foreach› loop instead of a copy-pasta for checking both arrays at
the same time.

URL:	https://leetcode.com/problems/find-the-prefix-common-array-of-two-arrays/
Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/find-the-prefix-common-array-of-two-arrays.cs

@@ -5,16 +5,12 @@ public class Solution {
 
         var (common, result) = (0, new int[n]);
         for (var i = 0; i < n; ++i) {
-            ++freqs[xs[i]];
-            if (freqs[xs[i]] == 2) {
-                // occurred in both
-                ++common;
-            }
-
-            ++freqs[ys[i]];
-            if (freqs[ys[i]] == 2) {
-                // occurred in both
-                ++common;
+            foreach (var x in new List<int>() { xs[i], ys[i] }) {
+                ++freqs[x];
+                if (freqs[x] == 2) {
+                    // occurred in both
+                    ++common;
+                }
             }
 
             result[i] = common;