[njngwn] WEEK 02 solutions (#2676)

* solve valid anagram

* solve climbing stairs

Author: njngwn

climbing-stairs/njngwn.py

@@ -0,0 +1,12 @@
+class Solution:
+    # dynamic programming with bottom-up
+    # Time Complexity: O(n)
+    # Space Complexity: O(1)
+    def climbStairs(self, n: int) -> int:
+        if n <= 2: return n
+        prev1, prev2 = 2, 1
+
+        for i in range(3, n+1):
+            prev1, prev2 = prev2 + prev1, prev1
+
+        return prev1

valid-anagram/njngwn.py

@@ -0,0 +1,7 @@
+from collections import Counter
+
+class Solution:
+    # Time Complexity: O(n), n: max(len(s), len(t))
+    # Space Complexity: O(k), k: number of letters
+    def isAnagram(self, s: str, t: str) -> bool:
+        return Counter(s) == Counter(t)