題目
https://leetcode.com/problems/generate-parentheses/
leetcode：22. Generate Parentheses 生成括號對

描述
For example, given n = 3, a solution set is:

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

import java.util.*;
class Solution {
    public List<String> generateParenthesis(int n) {
        List<String> result=new ArrayList<>();
        backTrack(n,result,new StringBuilder(),0,0);
        return result;
    }

    private void backTrack(int n,List<String> result,StringBuilder sb,int openCount,int closeCount) {
        if(sb.length()==2*n){
            result.add(sb.toString());
            return;
        }

        if(openCount<n){
            backTrack(n,result,sb.append("("),openCount+1,closeCount);
            sb.deleteCharAt(sb.length()-1);
            //openCount--;
        }

        if(closeCount<openCount){
            backTrack(n,result,sb.append(")"),openCount,closeCount+1);
            sb.deleteCharAt(sb.length()-1);
            //closeCount--;
        }
    }
}