搬个官方题解

class Solution {
    bool valid(const string& str) {//验证是否合法
        int balance = 0;
        for (char c : str) {
            if (c == '(') {
                ++balance;
            } else {
                --balance;
            }
            if (balance < 0) {
                return false;
            }
        }
        return balance == 0;
    }

    void generate_all(string& current, int n, vector<string>& result) {
        if (n == current.size()) {
            if (valid(current)) {
                result.push_back(current);
            }
            return;
        }
        current += '(';
        generate_all(current, n, result);
        current.pop_back();
        current += ')';
        generate_all(current, n, result);
        current.pop_back();
    }
public:
    vector<string> generateParenthesis(int n) {
        vector<string> result;
        string current;
        generate_all(current, n * 2, result);
        return result;
    }
};
class Solution {
    public List<String> generateParenthesis(int n) {
        List<String> ans = new ArrayList<String>();
        backtrack(ans, new StringBuilder(), 0, 0, n);
        return ans;
    }

    public void backtrack(List<String> ans, StringBuilder cur, int open, int close, int max) {
        if (cur.length() == max * 2) {
            ans.add(cur.toString());
            return;
        }
        if (open < max) {
            cur.append('(');
            backtrack(ans, cur, open + 1, close, max);
            cur.deleteCharAt(cur.length() - 1);
        }
        if (close < open) {
            cur.append(')');
            backtrack(ans, cur, open, close + 1, max);
            cur.deleteCharAt(cur.length() - 1);
        }
    }
}

作者：LeetCode-Solution
链接：https://leetcode-cn.com/problems/generate-parentheses/solution/gua-hao-sheng-cheng-by-leetcode-solution/
来源：力扣（LeetCode）
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