Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

For example, given n = 3, a solution set is:

"((()))", "(()())", "(())()", "()(())", "()()()"

没做出来，网上查的资料：

class Solution {
public:
    std::string tmp;
std::vector<std::string> ans;

void DFS(int left, int right, int n)
{
	if(left == right && left == n)
	{
		ans.push_back(tmp);
		return;
	}
	if(left < n)
	{
		tmp[left + right] = ‘(‘;
		DFS(left + 1, right, n);
	}
	if(right < left)
	{
		tmp[left + right] = ‘)‘;
		DFS(left, right + 1, n);
	}	
}
std::vector<std::string> generateParenthesis(int n)
{
	tmp.resize(n << 1);
	DFS(0, 0, n);
	return ans;
}
};