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:

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

难度：medium

题目：给定n对圆括号，写函数产生其所有格式良好的组合形式。

思路：右边括号数一定不能大于左边，然后利用递归来实现。

Runtime: 1 ms, faster than 100.00% of Java online submissions for Generate Parentheses.

class Solution {    public List
    
      generateParenthesis(int n) {        List
     
       result = new ArrayList<>();        generateParenthesis(0, 0, n, new StringBuilder(), result);        return result;    }        private void generateParenthesis(int left, int right, int n, StringBuilder sb, List
      
        result) {        if (left < right) {            return;        }        if (left == right && n == left) {            result.add(sb.toString());            return;        }                if (left < n && left >= right) {            sb.append("(");            generateParenthesis(left + 1, right, n, sb, result);            sb.deleteCharAt(sb.length() - 1);        }                if (right < left) {            sb.append(")");            generateParenthesis(left, right + 1, n, sb, result);            sb.deleteCharAt(sb.length() - 1);        }    }}