add code of 70

Author: Li Li

top-interview-questions/70. Climbing Stairs.cs

@@ -0,0 +1,30 @@
+public class Solution {
+    public int ClimbStairs(int n) {
+        if (n == 1) return 1;
+        if (n == 2) return 2;
+        int[] res = new int[n+1];
+        res[1] = 1;
+        res[2] = 2;
+        for (int i = 3; i <= n; i++) {
+            res[i] = res[i - 1] + res[i - 2];
+        }
+        return res[n];
+    }
+}
+
+// refactor to Fibonacci Number
+public class Solution {
+    public int ClimbStairs(int n) {
+        if (n == 1) {
+            return 1;
+        }
+        int first = 1;
+        int second = 2;
+        for (int i = 3; i <= n; i++) {
+            int third = first + second;
+            first = second;
+            second = third;
+        }
+        return second;
+    }
+}