leetcode 22-Generate Parentheses(medium)

class Solution {
    public List<String> generateParenthesis(int n) {
        List<String> list=new ArrayList<>();
        if(n==0) return list;
        generate(list, "", n, 0, 0);
        return list;
    }
    public void generate(List<String> list, String str, int n, int nleft, int index){
        if(index>=2*n){
            list.add(str);return;
        }
        if(nleft<n){
            generate(list, str+‘(‘, n, nleft+1, index+1);
        }
        if(index-nleft<nleft){
            generate(list, str+‘)‘, n, nleft, index+1);
        }
    }
}