cs: add «860. Lemonade Change»

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

cs/lemonade-change.cs

@@ -0,0 +1,51 @@
+public class Solution {
+    private enum Bill {
+        FIVE,
+        TEN,
+        TWENTY
+    }
+
+    private static Bill FromNumeric(int bill) => bill switch {
+        5 => Bill.FIVE,
+        10 => Bill.TEN,
+        20 => Bill.TWENTY,
+        _ => throw new ArgumentOutOfRangeException(nameof(bill), "Invalid bill"),
+    };
+
+    public bool LemonadeChange(int[] bills) {
+        var counters = new Dictionary<Bill, int> {
+            { Bill.FIVE, 0 },
+            { Bill.TEN, 0 },
+            { Bill.TWENTY, 0 }
+        };
+
+        foreach (var numericBill in bills) {
+            var bill = FromNumeric(numericBill);
+            ++counters[bill];
+
+            switch (bill) {
+                case Bill.FIVE:
+                    /* no-op */
+                    break;
+                case Bill.TEN:
+                    if (counters[Bill.FIVE] <= 0) {
+                        return false;
+                    }
+                    --counters[Bill.FIVE];
+                    break;
+                case Bill.TWENTY:
+                    if (counters[Bill.TEN] > 0 && counters[Bill.FIVE] > 0) {
+                        --counters[Bill.FIVE];
+                        --counters[Bill.TEN];
+                    } else if (counters[Bill.FIVE] >= 3) {
+                        counters[Bill.FIVE] -= 3;
+                    } else {
+                        return false;
+                    }
+                    break;
+            }
+        }
+
+        return true;
+    }
+}