【回溯】22. 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:

[
  "((()))",
  "(()())",
  "(())()",
  "()(())",
  "()()()"
]

解法：

比较intuitive：

class Solution {
public:
    vector<string> generateParenthesis(int n) {
        vector<string> ans;
        generate(ans,"",n,0);
        return ans;
    }
    void generate(vector<string>& ans,string str,int l,int r){
        if(l==0&&r==0){
            ans.emplace_back(str);
            return;
        }
        if(l>0){
            generate(ans,str+"(",l-1,r+1);
        }
        if(r>0){
            generate(ans,str+")",l,r-1);
        }
    }
};

很久没有写递归的代码了，特此记录。