题解 | #括号生成#

import java.util.*;


public class Solution {
    /**
     *
     * @param n int整型
     * @return string字符串ArrayList
     */
    public ArrayList<String> generateParenthesis (int n) {
        // write code here
        ArrayList<String> res = new ArrayList<>();
        parenthesis(0, 0, n, "", res);
        return res;
    }
    private void parenthesis(int left, int right, int n, String str,
                             ArrayList<String> res) {
        if (left == n && right == n) {
            res.add(str);
            return;
        }
        if (left < n) {
            parenthesis(left + 1, right, n, str + "(", res);
        }
        if (left > right) {
            parenthesis(left, right + 1, n, str + ")", res);
        }
    }
}