Create the-k-th-lexicographical-string-of-all-happy-strings-of-length-n.py

Author: kamyu104

Python/the-k-th-lexicographical-string-of-all-happy-strings-of-length-n.py

@@ -0,0 +1,20 @@
+# Time:  O(n)
+# Space: O(1)
+
+class Solution(object):
+    def getHappyString(self, n, k):
+        """
+        :type n: int
+        :type k: int
+        :rtype: str
+        """
+        base = 2**(n-1)
+        if k > 3*base:
+            return ""
+        result = [chr(ord('a')+(k-1)//base)]
+        while base > 1:
+            k -= (k-1)//base*base
+            base //= 2
+            result.append(('a' if result[-1] != 'a' else 'b') if (k-1)//base == 0 else
+                          ('c' if result[-1] != 'c' else 'b'))
+        return "".join(result)