adding backtracking-1 soln

W6_combination_sum.py

@@ -0,0 +1,27 @@
+class Solution:
+    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
+        self.result = []
+        self.helper(candidates, target, 0, [])
+        return self.result 
+
+    def helper(self, candidates, target, i, path):
+        if target < 0 or i == len(candidates):
+            return
+
+        if target == 0:
+            self.result.append(list(path)) 
+            return
+
+        # Don't Choose
+        self.helper(candidates, target, i + 1, path) # Deep copy path list
+
+        # Choose
+        path.append(candidates[i])
+
+        self.helper(candidates, target - candidates[i], i, path) # Deep copy path list
+
+        path.pop()
+
+# Time complexity: O(2 ^ (m+n))
+# Space complexity: O(n)
+