dfs 的一个关键是termination condition

public class Solution {    public ArrayList
     
       generateParenthesis(int n) {        ArrayList
      
        res = new ArrayList
       
        ();        if(n<1) return res;        dfs(n,n,res,new String());        return res;    }    private void dfs(int nl, int nr,ArrayList
        
          res, String item){        if(nr
         
          nl not working!!!        if(nr==0 && nl==0) res.add(item);                if(nl>0)            dfs(nl-1, nr, res, item+'(');        if(nr>0)            dfs(nl, nr-1, res, item+')');    }}