LeetCode 22. Generate Parentheses (Medium)

题目描述：

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

Example：

Given n = 3, a solution set is:[  "((()))",  "(()())",  "(())()",  "()(())",  "()()()"]

题目大意：给出n，生成所有包含n对括号的合法的括号串。

思路：模拟二叉树遍历，当左括号有剩余，添加左括号，当右括号数大于左括号，添加右括号。

c++代码：

class Solution {public:    vector<string> generateParenthesis(int n) {        dfs("", n, n);        return ans;    }    void dfs(string str, int left, int right)    {        if (left == 0 && right == 0)        {            ans.push_back(str);            return;        }        if (left > 0)            dfs(str + "(", left - 1, right);        if (left < right)            dfs(str + ")", left, right - 1);    }private:    vector<string> ans;};