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;
};