leetcode--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:

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

利用回溯算法：

如果左右括号都达到n，则push_back

如果左括号小于n，递归添加左括号的字符串

如果右括号小于左括号，递归添加右括号的字符串

class Solution {public:    vector<string> ret;    void bt(string strin, int left, int right, int n) {            if(left == n && right == n) {                ret.push_back(strin);            }            if(left < n) {                string new_str = strin + '(';                bt(new_str, left+1, right, n);            }            if(right < left) { //here, originally I use right < n && right < left, however, the first is redundant                string new_str = strin + ')';                bt(new_str, left, right + 1, n);            }    }        vector<string> generateParenthesis(int n) {        bt("", 0, 0, n);        return ret;    }};