22. 括号生成

题目：

算法思想：
回溯法：（深度优先遍历+剪枝）

代码：

class Solution {
   public List generateParenthesis(int n) {
        ArrayList res = new ArrayList<>();
        dfs("", n, n, res);
        return res;
    }

    public static void dfs(String temp, int left, int right, ArrayList res) {
        if (left == 0 && right == 0) {
            res.add(temp);
            return;
        }
        if (right < left) {
            return;
        }
        if (left != 0) {
            dfs(temp + "(", left - 1, right, res);
        }
        if (right != 0) {
            dfs(temp + ")", left, right - 1, res);
        }
        return;
    }
    
}