cs: add «58. Length of Last Word»

Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/length-of-last-word.cs

@@ -0,0 +1,25 @@
+public class Solution {
+    public int LengthOfLastWord(string s) {
+        var lastLength = 0;
+
+        var length = 0;
+        foreach (var c in s) {
+            if (char.IsWhiteSpace(c)) {
+                if (length != 0) {
+                    lastLength = length;
+                }
+
+                length = 0;
+                continue;
+            }
+
+            ++length;
+        }
+
+        if (length != 0) {
+            lastLength = length;
+        }
+
+        return lastLength;
+    }
+}