cs: add «714. Best Time to Buy and Sell Stock with Transaction Fee»

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

cs/best-time-to-buy-and-sell-stock-with-transaction-fee.cs

@@ -0,0 +1,13 @@
+public class Solution {
+    public int MaxProfit(int[] prices, int fee) {
+        var profit = 0;
+        var buyPrice = prices[0];
+
+        for (var i = 1; i < prices.Length; ++i) {
+            profit = Math.Max(profit, prices[i] - buyPrice - fee);
+            buyPrice = Math.Min(buyPrice, prices[i] - profit);
+        }
+
+        return profit;
+    }
+}