22. 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:
[ "((()))", "(()())", "(())()", "()(())", "()()()"]

public class Solution {
    
    List<String> res = new ArrayList<String>();
    public List<String> generateParenthesis(int n) {
        helper(n,n,"");
        return res;
    }
    
    public void helper(int left,int right,String tmp)
    {
        if(right<left)
           return;
        if(left==0&&right==0)
          {
              res.add(tmp);
              return;
          }
        if(left>0)
          helper(left-1,right,tmp+"(");
        if(right>left)
          helper(left,right-1,tmp+")");
    }
    
}