LeetCode 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 {
    String parenthes = "()";
    List<String> lastParenthesis=new ArrayList<>();
    public List<String> generateParenthesis(int n) {
            List<String> Parenthesis=new ArrayList<>();
        if (n==0) {
            return lastParenthesis;
        }
        lastParenthesis.add(parenthes);
        if (n==1) {    
            return lastParenthesis;
        }else {
            for (int i = 2; i <=n; i++) {
                for (String eleP : lastParenthesis) {
                    for (int j = 0; j < eleP.length(); j++) {
                        String newone=eleP.substring(0, j+1)+parenthes+eleP.substring(j+1);
                        if (!Parenthesis.contains(newone)) {
                            Parenthesis.add(newone);
                        }
                    }

                }
                    lastParenthesis.clear();
                    lastParenthesis.addAll(Parenthesis);
                    Parenthesis.clear();
            }
        }
        return lastParenthesis;
    }
}