cs: add «273. Integer to English Words»

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

cs/integer-to-english-words.cs

@@ -0,0 +1,51 @@
+public class Solution {
+    private static readonly List<(int value, string word)> NUMBERS = new List<(int value, string word)>() {
+        (1000000000, "Billion"),
+        (1000000, "Million"),
+        (1000, "Thousand"),
+        (100, "Hundred"),
+        (90, "Ninety"),
+        (80, "Eighty"),
+        (70, "Seventy"),
+        (60, "Sixty"),
+        (50, "Fifty"),
+        (40, "Forty"),
+        (30, "Thirty"),
+        (20, "Twenty"),
+        (19, "Nineteen"),
+        (18, "Eighteen"),
+        (17, "Seventeen"),
+        (16, "Sixteen"),
+        (15, "Fifteen"),
+        (14, "Fourteen"),
+        (13, "Thirteen"),
+        (12, "Twelve"),
+        (11, "Eleven"),
+        (10, "Ten"),
+        (9, "Nine"),
+        (8, "Eight"),
+        (7, "Seven"),
+        (6, "Six"),
+        (5, "Five"),
+        (4, "Four"),
+        (3, "Three"),
+        (2, "Two"),
+        (1, "One"),
+    };
+
+    public string NumberToWords(int num) {
+        if (num == 0) {
+            return "Zero";
+        }
+
+        foreach (var (value, word) in NUMBERS) {
+            if (num >= value) {
+                var prefix = (num >= 100) ? NumberToWords(num / value) + " " : "";
+                var suffix = (num % value == 0) ? "" : " " + NumberToWords(num % value);
+                return prefix + word + suffix;
+            }
+        }
+
+        return "";
+    }
+}