diff --git a/csharp/1370-increasing-decreasing-string.cs b/csharp/1370-increasing-decreasing-string.cs
new file mode 100644
index 000000000..d5151a5ba
--- /dev/null
+++ b/csharp/1370-increasing-decreasing-string.cs
@@ -0,0 +1,31 @@
+public class Solution {
+    public string SortString(string s) {
+        var charCounts = new Dictionary<char, int>();
+
+        foreach (var ch in s) {
+            if (charCounts.ContainsKey(ch)) {
+                charCounts[ch]++;
+            } else {
+                charCounts[ch] = 1;
+            }
+        }
+
+        var sortedKeys = charCounts.Keys.ToList();
+        sortedKeys.Sort();
+
+        var pattern = string.Concat(sortedKeys) + string.Concat(sortedKeys.AsEnumerable().Reverse());
+        var maxItem = charCounts.Values.Max();
+        var sample = string.Concat(Enumerable.Repeat(pattern, (maxItem / 2) + 1));
+
+        var result = "";
+
+        foreach (var ch in sample) {
+            if (charCounts.ContainsKey(ch) && charCounts[ch] > 0) {
+                result += ch;
+                charCounts[ch]--;
+            }
+        }
+
+        return result;
+    }
+}
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