Update combination_sum.py

Author: MaximSmolskiy

backtracking/combination_sum.py

@@ -53,13 +53,14 @@ def combination_sum(candidates: list, target: int) -> list:
         ...
     ValueError: Candidates list should not be empty
     """
-    path = []  # type: list[int]
-    answer = []  # type: list[int]
     if not candidates:
         raise ValueError("Candidates list should not be empty")
 
     if any(x < 0 for x in candidates):
         raise ValueError("All elements in candidates must be non-negative.")
+
+    path = []  # type: list[int]
+    answer = []  # type: list[int]
     backtrack(candidates, path, answer, target, 0)
     return answer