Generate Parentheses

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

For example, given n = 3, a solution set is:

  1. [
  2.   "((()))",
  3.   "(()())",
  4.   "(())()",
  5.   "()(())",
  6.   "()()()"
  7. ]

Solution:

  1. public class Solution {
  2.   public List<String> generateParenthesis(int n) {
  3.     List<String> res = new ArrayList<String>();
  4.     helper(n, 0, 0, "", res);
  5.     return res;
  6.   }
  8.   private void helper(int n, int left, int right, String sol, List<String> res) {
  9.     if (right == n) {
  10.       res.add(sol);
  11.       return;
  12.     }
  14.     if (left < n) {
  15.       helper(n, left + 1, right, sol + "(", res);
  16.     }
  18.     if (right < left) {
  19.       helper(n, left, right + 1, sol + ")", res);
  20.     }
  21.   }
  22. }