Update 0738.单调递增的数字，添加C#

Author: eeee0717

problems/0738.单调递增的数字.md

@@ -392,6 +392,30 @@ impl Solution {
     }
 }
 ```
+### C#
+```csharp
+public class Solution
+{
+    public int MonotoneIncreasingDigits(int n)
+    {
+        char[] s = n.ToString().ToCharArray();
+        int flag = s.Length;
+        for (int i = s.Length - 1; i > 0; i--)
+        {
+            if (s[i - 1] > s[i])
+            {
+                flag = i;
+                s[i - 1]--;
+            }
+        }
+        for (int i = flag; i < s.Length; i++)
+        {
+            s[i] = '9';
+        }
+        return int.Parse(new string(s));
+    }
+}
+```
 
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