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:

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

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注意：当使用引用穿参后，递归调用后，一定要进行pop_back

class Solution {
public:
    vector<string> generateParenthesis(int n) {
        vector<string> res;
        string s;
        helper(res, s, n, n);
        return res;
        
    }
    void helper(vector<string> &res, string &s, int left, int right){
        if(left == 0 && right == 0){
            res.push_back(s);
            return;
        }
        if(left > 0) { s.push_back('('); helper(res, s, left - 1, right); s.pop_back();
            
        }
        if(left < right) {s.push_back(')'); helper(res, s, left, right - 1);s.pop_back();}
    }
};