【leetcode】22. Generate Parentheses

/**
* @author          johnsondu
* @problem         Generate Parentheses
* @url             https://leetcode.com/problems/generate-parentheses/ * @strategy        dfs, only if left "(" less than right ")",  then it is legal.
* @status          Accepted, runtime beats 34.01% of cpp submissions. 0ms
* @time            19:46, Nov 3th 2015
*/

class Solution {
public:
void dfs(string tmp, int lf, int rt, vector<string>& ans)
{
if(lf == 0 && rt == 0) {
ans.push_back(tmp);
}
if(lf != 0 && lf <= rt) {
dfs(tmp + "(", lf - 1, rt, ans);
}
if(lf < rt) {
dfs(tmp + ")", lf, rt - 1, ans);
}
}

vector<string> generateParenthesis(int n) {
vector<string> ans;
if(n == 0) return ans;
dfs("(", n-1, n, ans);
return ans;
}
};