Create 078_subsets.md

Author: Joseph Luce

leetcode/medium/078_subsets.md

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+# 78. Subsets
+
+## Recursive Solution
+- Runtime: O(2^N)
+- Space: O(N)
+- N = Number of elements in array
+
+Using the ability for recursion to backtrack will allow us to populate the result.
+During each recursion, we will loop through the given array, during this loop, the number represent a choosen number for the subset.
+The numbers that were not choosen yet will be passed to the next recursion to be choosen again, hence, creating the result.
+By keeping a stack, we can use it as we traverse/recur, we append prior and pop after using this stack to add to the result of subsets.
+
+You can visually map this out using this example:
+
+Input: [1,2,3]
+
+1. R1: Select 1 -> [1]
+2. R2: Select 2 -> [1,2]
+3. R3: Select 3 -> [1,2,3] -> Pop 3 -> Done
+5. R2: Pop 2 -> Select 3 -> [1,3] -> Pop 3 -> Done
+7. R1: Pop 1 -> Select 2 -> [2]
+8. R2: Select 3 -> [2,3] -> Pop 3 -> Done
+9. R1: Select 3 -> [3] -> Pop 3 -> Done
+
+```
+class Solution:
+    def subsets(self, nums: List[int]) -> List[List[int]]:
+        results = list()
+        results.append([])
+        self.subset_helper(nums, [], results)
+        return results
+        
+    def subset_helper(self, nums, curr_result, results):
+        for index, n in enumerate(nums):
+            curr_result.append(n)
+            results.append([str(n) for n in curr_result])
+            self.subset_helper(nums[index+1:], curr_result, results)
+            curr_result.pop()
+```