add solution: combination-sum

Author: dusunax

combination-sum/dusunax.py

@@ -0,0 +1,39 @@
+'''
+# 39. Combination Sum
+
+Backtracking for find combinations.
+
+## Time and Space Complexity
+
+```
+TC: O(n^2)
+SC: O(n)
+```
+
+#### TC is O(n^2):
+- iterating through the list in backtracking recursion to find combinations. = O(n^2)
+
+#### SC is O(n):
+- using a list to store the combinations. = O(n)
+'''
+
+class Solution:
+    # Backtracking = find combination
+    # candidate is distinct & can use multiple times.
+    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
+        result = []  
+
+        def backtrack(currIdx, remain, combination):
+            if(remain == 0):
+                result.append(combination[:])
+                return
+            if(remain < 0):
+                return
+
+            for i in range(currIdx, len(candidates)):
+                combination.append(candidates[i])
+                backtrack(i, remain - candidates[i], combination) 
+                combination.pop()
+
+        backtrack(0, target, [permutations])
+        return result