[DFS] Add a solution to Factor Combinations

DFS/FactorCombinations.swift

@@ -0,0 +1,39 @@
+/**
+ * Question Link: https://leetcode.com/problems/factor-combinations/
+ * Primary idea: Classic Depth-first Search
+ * 
+ * Time Complexity: O(n^n), Space Complexity: O(2^n - 1)
+ *
+ */
+
+class FactorCombinations {
+    func getFactors(_ n: Int) -> [[Int]] {
+        var res = [[Int]]()
+        var path = [Int]()
+
+        dfs(&res, &path, n, 2)
+
+        return res
+    }
+
+    private func dfs(_ res: inout [[Int]], _ path: inout [Int], _ n: Int, _ start: Int) {
+        if n == 1 {
+            if path.count > 1 {
+                res.append(Array(path))
+            }
+            return
+        }
+
+        if start > n {
+            return
+        }
+
+        for i in start ... n {
+            if n % i == 0 {
+                path.append(i)
+                dfs(&res, &path, n / i, i)
+                path.removeLast()
+            }
+        }
+    }
+}