添加0509斐波那契数C#版本

problems/0070.爬楼梯.md

@@ -425,5 +425,25 @@ object Solution {
 }
 ```
 
+### C#
+
+```c#
+public class Solution {
+    public int ClimbStairs(int n) {
+        if(n<=2) return n;
+        int[] dp = new int[2] { 1, 2 };
+        for (int i = 3; i <= n; i++)
+        {
+            int temp = dp[0] + dp[1];
+            dp[0] = dp[1];
+            dp[1] = temp;
+        }
+        return dp[1];
+    }
+}
+```
+
+
+
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