22. Generate Parentheses

public class Solution {
    public List<String> generateParenthesis(int n) {
        // 左括号数 >= 有括号数
        List<String> list = new ArrayList<String>();
        StringBuffer sb = new StringBuffer();

        recursive(0, 0, n, sb, list);

        return list;
    }

    private static void recursive(int left, int right, int n, StringBuffer sb, List<String> list) {

        if(left < right)    return;

        if(left == n && right == n) {
            list.add(sb.toString());
            return;
        }

        if(left < n) {
            StringBuffer newSB = new StringBuffer(sb.toString());
            newSB.append("(");
            recursive(1+left, right, n, newSB, list);
        }

        if(right < n) {
            StringBuffer newSB = new StringBuffer(sb.toString());
            newSB.append(")");
            recursive(left, 1+right, n, newSB, list);
        }
    }
}