力扣上的全排列报错？

Question

执行完成，耗时：N/A
1
java.lang.NullPointerException: Cannot store to int array because "[]" is null
at line 13, Solution.permute
at line 82, MainClass.main

class Solution {
    public List<List<Integer>> permute(int[] nums) {
        int i,len=1;
        for(i=nums.length;i>0;i--)len=len*i;
       List<List<Integer>> end1=new ArrayList<List<Integer>>();
       int[][] end=new int[len][];
       for(int d=0;d<nums.length;d++){
           i=0;
       for(int j=0;j<nums.length;j++){
       for(int k=1;k<nums.length;k++)
       {
         System.out.print(nums[j]);
 13行          end[i][d]=nums[j];
           System.out.print(end[i][d]);
           i++;
       }
       }
       }
       for(i=0;i<len;i++)
       {
        List<Integer> list = new ArrayList<Integer>();
         for(int s=0;s<nums.length;s++){
         list.add(end[i][s]);
         }
         end1.add(list);
       }
       return end1;
    }
}

public class MainClass {
    public static int[] stringToIntegerArray(String input) {
        input = input.trim();
        input = input.substring(1, input.length() - 1);
        if (input.length() == 0) {
          return new int[0];
        }
    
        String[] parts = input.split(",");
        int[] output = new int[parts.length];
        for(int index = 0; index < parts.length; index++) {
            String part = parts[index].trim();
            output[index] = Integer.parseInt(part);
        }
        return output;
    }
    
    public static String integerArrayListToString(List<Integer> nums, int length) {
        if (length == 0) {
            return "[]";
        }
    
        String result = "";
        for(int index = 0; index < length; index++) {
            Integer number = nums.get(index);
            result += Integer.toString(number) + ", ";
        }
        return "[" + result.substring(0, result.length() - 2) + "]";
    }
    
    public static String integerArrayListToString(List<Integer> nums) {
        return integerArrayListToString(nums, nums.size());
    }
    
    public static String int2dListToString(List<List<Integer>> nums) {
        StringBuilder sb = new StringBuilder("[");
        for (List<Integer> list: nums) {
            sb.append(integerArrayListToString(list));
            sb.append(",");
        }
    
        sb.setCharAt(sb.length() - 1, ']');
        return sb.toString();
    }  

 
 75行   public static void main(String[] args) throws IOException {
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        String line;
        while ((line = in.readLine()) != null) {
            int[] nums = stringToIntegerArray(line);
            
            List<List<Integer>> ret = new Solution().permute(nums);
            
            String out = int2dListToString(ret);
            
            System.out.print(out);
        }
    }
}