保证左边‘（’的数量始终大于等于右边的‘）’数量，可以考虑回溯法
class Solution {
public:
vector<string> generateParenthesis(int n) {
vector<string>ans;
dfs("",0,0,n,ans);
return ans;
}
void dfs(string out,int left,int right,int n,vector<string>&ans)
{
if(left>n||right>n)return;
if(right==n){
ans.push_back(out);
return;
}
dfs(out+"(",left+1,right,n,ans);
if(left>right)dfs(out+")",left,right+1,n,ans);
}
};
