Generate Parentheses

    题意是要求生成匹配的括号结果。假如给定n = 3，需要生成3对括号的有效组合，即"((()))", "(()())", "(())()", "()(())", "()()()"。

void unguarded_generate(vector<string> &result, string curr, int m, int n)
{
    if (m == 0 && n == 0)
    {
        result.push_back(curr);
    }
    else
    {
        if (m != 0)
        {
            unguarded_generate(result, curr + "(", m - 1, n);
        }

        if (m < n && n != 0)
        {
            unguarded_generate(result, curr + ")", m, n - 1);
        }
    }
}

vector<string> generateParenthesis(int n)
{
    vector<string> ret;

    if (n > 0)
    {
        unguarded_generate(ret, string(), n, n);
    }

    return ret;
}