22.Generate Parenthesis

class Solution {
    public List<String> generateParenthesis(int n) {
        List<String>st=new ArrayList<>();
        rec(n,st,0,0,"");
        return st;
    }
    static void rec(int n,List<String>li,int i,int j,String p){
        if(j==i && i==n){
            li.add(p);
            return;
        }
       if(i<n){ 
        rec(n,li,i+1,j,p+'(');
       }
       if(j<i){
        rec(n,li,i,j+1,p+')');
       }
    }
}