生成括号
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:
["((()))","(()())","(())()","()(())","()()()" ]
方法解析：跟上道题目一样，使用的是回溯法
终止条件：括号数量 等于 2*n
递归参数：当open左括号小于n，追加左括号，左括号数量+1
当close右括号小于open，追加右括号，右括号数量+1；
class Solution {
public:vector<string> res;void backtrace(string s,int open,int close,int max){if(s.length() == max*2) {res.push_back(s); return;}if(open < max){backtrace(s+"(",open+1,close,max); }if(close < open){backtrace(s+")",open,close+1,max); }}vector<string> generateParenthesis(int n) {if(n ==0) return res;backtrace("",0,0,n);return res;}
};