rs: add “70. Climbing Stairs”

Signed-off-by: Matej Focko <me@mfocko.xyz>

rs/climbing-stairs.rs

@@ -0,0 +1,16 @@
+impl Solution {
+    pub fn climb_stairs(n: i32) -> i32 {
+        if n < 3 {
+            return n;
+        }
+
+        let mut ways = vec![0, 1, 2];
+        for k in 3..=n {
+            ways.remove(0);
+            ways.push(ways[0] + ways[1]);
+        }
+
+        ways[2]
+    }
+}
+