Add climb stairs solution

Author: hyunjung-choi

climbing-stairs/hyunjung-choi.kt

@@ -0,0 +1,16 @@
+class Solution {
+    fun climbStairs(n: Int): Int {
+        if (n <= 2) return n
+
+        var prev2 = 1
+        var prev1 = 2
+
+        for (i in 3..n) {
+            val current = prev1 + prev2
+            prev2 = prev1
+            prev1 = current
+        }
+
+        return prev1
+    }
+}