题目描述：

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: "((()))", "(()())", "(())()", "()(())", "()()()"

　　这题没做出来，惭愧。

solution：

void dfs(vector
     
       &result,string str,int left,int right){    if(left > right)        return;    if (left == 0 && right == 0)    {        result.push_back(str);        return;    }    if(left > 0)        dfs(result, str+"(", left-1, right);    if(right > 0)        dfs(result, str+")", left, right-1);}vector
      
        generateParenthesis(int n) {    vector
       
         res;    if(n < 1)         return res;    dfs(res, "", n, n);    return res;}
       
      
     

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