[DFS] Add solution to Remove Invalid Parentheses

DFS/RemoveInvalidParentheses.swift

@@ -0,0 +1,46 @@
+/**
+ * Question Link: https://leetcode.com/problems/remove-invalid-parentheses/
+ * Primary idea: Remove ) when the string is invalid, add to result when string is valid, 
+ *               and do the same thing for the reversed one
+ * 
+ * Time Complexity: O(n!), Space Complexity: O(n)
+ *
+ */
+
+class RemoveInvalidParentheses {
+    func removeInvalidParentheses(_ s: String) -> [String] {
+        var res = [String](), s = Array(s.characters)
+
+        dfs(&res, s, 0, 0, (Character("("), Character(")")))
+
+        return res
+    }
+
+    private func dfs(_ res: inout [String], _ s: [Character], _ lastI: Int, _ lastJ: Int, _ parens: (Character, Character)) {
+        var stack = 0, s = s
+
+        for i in lastI ..< s.count {
+            if s[i] == parens.0 {
+                stack += 1
+            }
+            if s[i] == parens.1 {
+                stack -= 1
+            }
+
+            if stack < 0 {
+                for j in lastJ ... i {
+                    if s[j] == parens.1 && (j == lastJ || s[j - 1] != parens.1) {
+                        dfs(&res, Array(s[0 ..< j] + s[j + 1 ..< s.count]), i, j, parens)
+                    }
+                }
+                return
+            }
+        }
+
+        if parens.0 == "(" {
+            dfs(&res, s.reversed(), 0, 0, (Character(")"), Character("(")))
+        } else {
+            res.append(String(s.reversed()))
+        }
+    }
+}