kt: add «70. Climbing Stairs»

URL:	https://leetcode.com/problems/climbing-stairs/
Signed-off-by: Matej Focko <me@mfocko.xyz>

kt/climbing-stairs.kt

@@ -0,0 +1,11 @@
+class Solution {
+    fun climbStairs(n: Int): Int =
+        when {
+            n < 3 -> n
+            else ->
+                (3..n).fold(Triple(0, 1, 2)) { acc, _ ->
+                    val (_, y, z) = acc
+                    Triple(y, z, y + z)
+                }.third
+        }
+}