Fix the time and space complexity

Author: Yi Gu

DFS/CombinationSum.swift

@@ -2,7 +2,7 @@
  * Question Link: https://leetcode.com/problems/combination-sum/
  * Primary idea: Classic Depth-first Search
  * 
- * Time Complexity: O(n^n), Space Complexity: O(2^n - 1)
+ * Time Complexity: O(2^n), Space Complexity: O(n)
  *
  */
 

DFS/CombinationSumII.swift

@@ -2,7 +2,7 @@
  * Question Link: https://leetcode.com/problems/combination-sum-ii/
  * Primary idea: Classic Depth-first Search
  * 
- * Time Complexity: O(n^n), Space Complexity: O(2^n - 2)
+ * Time Complexity: O(2^n), Space Complexity: O(n)
  *
  */
 

DFS/CombinationSumIII.swift

@@ -2,7 +2,7 @@
  * Question Link: https://leetcode.com/problems/combination-sum-iii/
  * Primary idea: Classic Depth-first Search
  * 
- * Time Complexity: O(n^n), Space Complexity: O(nCk)
+ * Time Complexity: O(2^n), Space Complexity: O(n)
  *
  */
 

DFS/Combinations.swift

@@ -2,7 +2,7 @@
  * Question Link: https://leetcode.com/problems/combinations/
  * Primary idea: Classic Depth-first Search, another version of Subsets
  * 
- * Time Complexity: O(n * 2^n), Space Complexity: O(n * 2^n)
+ * Time Complexity: O(2^n), Space Complexity: O(n)
  *
  */
 

README.md

@@ -271,14 +271,14 @@
 ## Depth-first search
 | Title | Solution | Difficulty | Time | Space |
 | ----- | -------- | ---------- | ---- | ----- |
-[Permutations](https://leetcode.com/problems/permutations/)| [Swift](./DFS/Permutations.swift)| Medium| O(n^n)| O(n)|
-[Permutations II](https://leetcode.com/problems/permutations-ii/)| [Swift](./DFS/PermutationsII.swift)| Medium| O(n^n)| O(n)|
+[Permutations](https://leetcode.com/problems/permutations/)| [Swift](./DFS/Permutations.swift)| Medium| O(2^n)| O(n)|
+[Permutations II](https://leetcode.com/problems/permutations-ii/)| [Swift](./DFS/PermutationsII.swift)| Medium| O(2^n)| O(n)|
 [Subsets](https://leetcode.com/problems/subsets/)| [Swift](./DFS/Subsets.swift)| Medium| O(n^n)| O(n)|
-[Subsets II](https://leetcode.com/problems/subsets-ii/)| [Swift](./DFS/SubsetsII.swift)| Medium| O(n^n)| O(n)|
-[Combinations](https://leetcode.com/problems/combinations/)| [Swift](./DFS/Combinations.swift)| Medium| O(n^n)| O(n)|
-[Combination Sum](https://leetcode.com/problems/combination-sum/)| [Swift](./DFS/CombinationSum.swift)| Medium| O(n^n)| O(2^n - 1)|
-[Combination Sum II](https://leetcode.com/problems/combination-sum-ii/)| [Swift](./DFS/CombinationSumII.swift)| Medium| O(n^n)| O(2^n - 2)|
-[Combination Sum III](https://leetcode.com/problems/combination-sum-iii/)| [Swift](./DFS/CombinationSumIII.swift)| Medium| O(n^n)| O(nCk)|
+[Subsets II](https://leetcode.com/problems/subsets-ii/)| [Swift](./DFS/SubsetsII.swift)| Medium| O(2^n)| O(n)|
+[Combinations](https://leetcode.com/problems/combinations/)| [Swift](./DFS/Combinations.swift)| Medium| O(2^n)| O(n)|
+[Combination Sum](https://leetcode.com/problems/combination-sum/)| [Swift](./DFS/CombinationSum.swift)| Medium| O(2^n)| O(n)|
+[Combination Sum II](https://leetcode.com/problems/combination-sum-ii/)| [Swift](./DFS/CombinationSumII.swift)| Medium| O(2^n)| O(n)|
+[Combination Sum III](https://leetcode.com/problems/combination-sum-iii/)| [Swift](./DFS/CombinationSumIII.swift)| Medium| O(2^n)| O(n)|
 [Letter Combinations of a Phone Number](https://leetcode.com/problems/letter-combinations-of-a-phone-number/)| [Swift](./DFS/LetterCombinationsPhoneNumber.swift)| Medium| O(4^n)| O(n)|
 [Factor Combinations](https://leetcode.com/problems/factor-combinations/)| [Swift](./DFS/FactorCombinations.swift)| Medium| O(n^n))| O(2^n - 1)|
 [Strobogrammatic Number II](https://leetcode.com/problems/strobogrammatic-number-ii/)| [Swift](./DFS/StrobogrammaticNumberII.swift)| Medium| O(m^n)| O(n)|