public class Solution {

    private List<String> list = new ArrayList<String>();  

    public List<String> generateParenthesis(int n) {

        run("", n, 0);

        return list;

    }  

    // l 为剩余左括号数，r为剩余未匹配的右括号数目

    public void run(String str, int l, int r) {

        if (l == 0 && r == 0) {

            list.add(str);

            return;  //跳出run方法

        }

        if (l > 0) {

            String s = str + "(";

            run(s, l-1, r+1);

        }

        if (r > 0) {

            String s = str + ")";

            run(s, l, r-1);

        }

    }

}