<p>Given <code>n</code> pairs of parentheses, write a function to <em>generate all combinations of well-formed parentheses</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<pre><strong>Input:</strong> n = 3
<strong>Output:</strong> ["((()))","(()())","(())()","()(())","()()()"]
</pre><p><strong class="example">Example 2:</strong></p>
<pre><strong>Input:</strong> n = 1
<strong>Output:</strong> ["()"]
</pre>
<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= n &lt;= 8</code></li>
</ul>

class Solution {
public:
    vector<string>ans;
    void gen(int left,int right,string s){
        if(left<0 || right<0){
            return;
        }
        if(left == 0 &&  right == 0){
            ans.push_back(s);
        }
        gen(left-1,right,s+'(');
        if(left<right){
            gen(left,right-1,s+')');
        }
    }
    vector<string> generateParenthesis(int n) {
        gen(n,n,"");
        return ans;
    }
};
16956