［LeetCode］Generate Parentheses

解題思路：

BackTracking。層層深入 

class Solution {public:    vector<string> generateParenthesis(int n) {        vector<string> ret;        search("", 0, n, ret);        return ret;    }    void search(string str, int leftNum, int n, vector<string> &ret){        if (n*2 == str.length()) {            ret.push_back(str);        }else{            if (leftNum == n){                search(str+")", leftNum, n, ret);            }else{                if (str.length() == leftNum*2){                    search(str+"(", leftNum+1, n, ret);                }else{                    search(str+"(", leftNum+1, n, ret);                    search(str+")", leftNum, n, ret);                }            }        }    }};