題目描述：

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:

"((()))", "(()())", "(())()", "()(())", "()()()"

思路解析：

• left代表左括號剩餘的個數，right代表右括號剩餘的個數
• 如果左括號大於右括號的個數，就結束了
• 如果都為0，就可以加入到ArrayList中

程式碼：

import java.util.*;
public class Solution {
    public ArrayList<String> generateParenthesis(int n) {
        ArrayList<String> res = new ArrayList<String>();
        if(n<0)
            return res;
        String item = new String();
        helper(n,n,item,res);
        return res;
    }
   public void helper(int left,int right,String item,ArrayList<String> res){
       if(left>right)
           return ;
       if(left==0 && right==0){
           res.add(new String(item));
           return;
       }
       if(left>0){
           helper(left-1,right,item+'(',res);
       }
       if(right>0)
           helper(left,right-1,item+')',res);
   }
}