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> res = new ArrayList<>();
    public ArrayList<String> generateParenthesis (int n) {
        // write code here
        if (n <= 0) return res;
        StringBuilder strack = new StringBuilder();
        backstrack(n, 0, 0, strack);
        return res;
    }
    private void backstrack(int n, int left, int right, StringBuilder strack) {
        if (strack.length() == n * 2) {
            res.add(strack.toString());
            return;
        }
        if (left < n) {
            strack.append('(');
            backstrack(n, left + 1, right, strack);
            strack.deleteCharAt(strack.length() - 1);
        }
        if (left > right) {
            strack.append(')');
            backstrack(n, left , right+1, strack);
            strack.deleteCharAt(strack.length() - 1);
        }
    }
}