Add a solution to combination-sum problem.

Author: KyrieJ95

combination-sum/joon.py

@@ -0,0 +1,34 @@
+from typing import List
+from typing import Set
+from typing import Tuple
+
+
+class Solution:
+    target: int
+    candidates: List[int]
+    answers: Set[Tuple[int]]
+
+    # Time: O(k^N)
+    # Space: O(N^2)
+    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
+        self.target = target
+        self.candidates = candidates
+        self.answers = set()
+        self.findAnswers(0, [])
+        return list(self.answers)
+
+    def findAnswers(self, currentSum: int, nums: List[int]):
+        assert currentSum < self.target, "Given sum should be smaller than the target."
+        for num in self.candidates:
+            if currentSum + num < self.target:
+                # 1. Continue finding.
+                # Time: O(k^N)
+                # Space: O(N^2). Max total size of all "nums" = 1 + 2 + ... + N.
+                self.findAnswers(currentSum + num, nums + [num])
+            if currentSum + num > self.target:
+                # 2. Stop finding.
+                continue
+            if currentSum + num == self.target:
+                # 3. Add the answer.
+                self.answers.add(tuple(sorted(list(nums + [num]))))
+        return