cs: add «3042. Count Prefix and Suffix Pairs I»

URL:	https://leetcode.com/problems/count-prefix-and-suffix-pairs-i/
Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/count-prefix-and-suffix-pairs-i.cs

@@ -0,0 +1,18 @@
+public class Solution {
+    private bool IsPrefixAndSuffix(string str1, string str2) =>
+        str1.Length <= str2.Length && str2.StartsWith(str1) && str2.EndsWith(str1);
+
+    public int CountPrefixSuffixPairs(string[] words) {
+        var count = 0;
+
+        for (var i = 0; i < words.Length; ++i) {
+            for (var j = i + 1; j < words.Length; ++j) {
+                if (IsPrefixAndSuffix(words[i], words[j])) {
+                    ++count;
+                }
+            }
+        }
+
+        return count;
+    }
+}