题解 | #括号生成#

import java.util.*;


public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param n int整型
     * @return string字符串ArrayList
     */
    ArrayList<String> result;
    public ArrayList<String> generateParenthesis (int n) {
        // write code here
        result = new ArrayList<String>();
        StringBuilder sb = new StringBuilder();
        solution(sb, n, n);
        return result;
    }
    public void solution(StringBuilder s, int left, int right) {
        if (left == 0 && right == 0) {
            result.add(s.toString());
            return;
        } else {
            if (left == right) {
                s.append("(");
                left--;
                solution(s, left, right);
                s.deleteCharAt(s.length() - 1);
                left++;
            } else if (left < right) {
                if (left != 0) {
                    s.append("(");
                    left--;
                    solution(s, left, right);
                    s.deleteCharAt(s.length() - 1);
                    left++;
                }
                if (right != 0) {
                    s.append(")");
                    right--;
                    solution(s, left, right);
                    s.deleteCharAt(s.length() - 1);
                    right++;
                }
            }
        }
    }
}