Create week6 22. Generate Parentheses

Author: andysli6590

week6 22. Generate Parentheses

@@ -0,0 +1,25 @@
+public class Solution {
+    public List<String> generateParenthesis(int n) {
+        List<String> res = new ArrayList<>();
+        helper (res, new String(), n, 0);
+        return res;
+    }
+    public void helper (List<String> res, String str, int n, int counter){
+        if(counter == 0 && str.length() == 2 * n){
+            res.add(new String(str));
+            return;
+        }
+        if(counter < 0 || counter > n || str.length() > 2*n){
+            return;
+        }
+        if(counter > 0 && counter < n){
+            helper(res, str + "(", n, counter + 1);
+            helper(res, str + ")", n, counter - 1);
+        }else if (counter == 0){
+            helper(res, str + "(", n, counter + 1);
+        }else {
+            helper(res, str + ")", n, counter - 1);
+        }
+        return;
+    }
+}