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
     */
    public ArrayList<String> generateParenthesis (int n) {
        // write code here
        String str = new String();
        ArrayList<String> ans = new ArrayList<>();
        callBack(ans,str,n,0);
        return ans;
    public void callBack(ArrayList<String> ans,String path,int left,int right){
        if(left == 0 && right == 0){
            ans.add(path);
            return;
        if(left>0){
            path = path + "(";
            int length = path.length();
            callBack(ans,path,left - 1,right + 1);
            path = path.substring(0,length-1);
            
        }
        if(right > 0){
            path = path + ")";
            int length = path.length();
            callBack(ans,path,left,right - 1);
            path = path.substring(0,length-1);
        }
    }
}