[LeetCode]Generate Parentheses

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;
}
};