add code of 22

Author: Li Li

top-interview-questions/22. Generate Parentheses.cs

@@ -0,0 +1,23 @@
+/*
+In general, for each bit, we can choose either "(" or ")", but in this problem.
+To get paired solution, for each bit site, there are some restrictions.
+To add "(": there must be some "(" remaining.
+To add ")": 1, there must be some "(" remaining.
+            2, previous acount of "(" must be more than ")".
+            1 + 2 => remainning "(" less than ")"
+
+ */
+public class Solution {
+    public IList<string> GenerateParenthesis(int n) {
+        IList<string> result = new List<string>();
+        if (n > 0) DFSHelper(result, "", n, n);
+        return result;
+    }
+    private void DFSHelper(IList<string> result, string s, int left, int right) {
+        if (left == 0 && right == 0) result.Add(s);
+        // has remaining "("
+        if (left > 0) DFSHelper(result, s + "(", left - 1, right);
+        // has more ")" remain
+        if (right > 0 && right > left) DFSHelper(result, s + ")", left, right - 1);
+    }
+}