题解 | #括号生成#

import java.util.*;


public class Solution {

    private ArrayList<String> res = new ArrayList<>();

    /**
     * 
     * @param n int整型 
     * @return string字符串ArrayList
     */
    public ArrayList<String> generateParenthesis (int n) {
        // write code here
        if (n < 1) {
            return res;
        }
        backtrack(new LinkedList<>(), n, n);
        return res;
    }

    private void backtrack(LinkedList<Character> list, int left, int right) {
        if (left > right || left < 0 || right < 0) {
            return;
        }
        if (left == 0 && right == 0) {
            res.add(list2Str(list));
            return;
        }

        list.add('(');
        backtrack(list, left - 1, right);
        list.removeLast();

        list.add(')');
        backtrack(list, left, right - 1);
        list.removeLast();
    }

    private String list2Str(LinkedList<Character> list) {
        StringBuilder sb = new StringBuilder();
        for (char ch : list) {
            sb.append(ch);
        }
        return sb.toString();
    }
}