pdd笔试题种树

import java.util.Scanner;
public class Main {
    private static boolean flag  = false;
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int[] A = new int[n + 1];
        int sum = 0;
        A[0] = 0;
        int i = 1;
        while (in.hasNextInt()) {//注意while处理多个case              int a = in.nextInt();
            int b = in.nextInt();
            A[i++] = b;
            sum += b;
        }
        int[] res = new int[sum];
        backtrace(A, res, 0, sum);
        if(flag == false){
            System.out.println("-");
        }else{
            for(int j = 0; j < res.length; j++){
                System.out.print(res[j]+" ");
            }
        }
            
    }
    public static void backtrace(int[] A, int[] res, int index, int n){
        if(index == n){
            flag = true;
            return;
        }
        if(flag == false){
            for(int i = 0; i < A.length; i++){
                if(A[i] != 0){
                    if(index == 0 || res[index - 1] != i){
                        res[index++] = i;
                        A[i]--;                   
                        backtrace(A, res, index, n);
                        if(flag == true){
                            break;
                        }
                        index--;
                        A[i]++;
                    }
                }
            }
        }
    } 
}

最后一个case超时，待改进