22. 括号生成

[
  "((()))",
  "(()())",
  "(())()",
  "()(())",
  "()()()"
]

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

    public void dfs(List<String> list, String st, int left, int right) {
        //left>right表示组合字符串中已存在(数量要小于）数量，肯定不可能组合成功
        if (left > right) {
            return;
        }
        if (left == 0 && right == 0) {
            list.add(st);
        }
        if (left > 0) {
            dfs(list, st + "(", left - 1, right);
        }
        if (right > 0) {
            dfs(list, st + ")", left, right - 1);
        }
    }
}