cs: add “724. Find Pivot Index”

Signed-off-by: Matej Focko <mfocko@redhat.com>

cs/find-pivot-index.cs

@@ -0,0 +1,18 @@
+public class Solution {
+    public int PivotIndex(int[] nums) {
+        var fromLeft = 0;
+        var fromRight = nums.Sum();
+
+        for (var i = 0; i < nums.Length; ++i) {
+            fromRight -= nums[i];
+
+            if (fromLeft == fromRight) {
+                return i;
+            }
+
+            fromLeft += nums[i];
+        }
+
+        return -1;
+    }
+}