Here is the DP question,
Given a ‘N’ * ’M’ maze with obstacles, count and return the number of unique paths to reach the right-bottom cell from the top-left cell. A cell in the given maze has a value ‘-1’ if it is a blockage or dead-end, else 0. From a given cell, we are allowed to move to cells (i+1, j) and (i, j+1) only. Since the answer can be large, print it modulo 10^9 + 7.
For Example :

Consider the maze below :
0 0 0
0 -1 0
0 0 0

There are two ways to reach the bottom left corner –

(1, 1) -> (1, 2) -> (1, 3) -> (2, 3) -> (3, 3)
(1, 1) -> (2, 1) -> (3, 1) -> (3, 2) -> (3, 3)

Hence the answer for the above test case is 2.

**This is my solution using 2d Arrays.
**

import java.util.*;

public class Solution {

public static int f(int i, int j, ArrayList<ArrayList<Integer>> mat,int [][]dp){

    if(i == 0 && j == 0)
    return 1;
  if((i < 0) || j < 0 || (mat.get(i).get(j)) == -1)
    return 0;
    if(dp[i][j] != -1)
    return dp[i][j];
   
   
    return dp[i][j] =  f(i-1,j,mat,dp)%1000000007+f(i,j-1,mat,dp)%1000000007;
   

}

static int mazeObstacles(int n, int m, ArrayList<ArrayList<Integer>> mat) {
    int dp[][] = new int[n][m];
    
    // Initialize the dp array with -1
    for (int row[] : dp)
        Arrays.fill(row, -1);
   return f(n-1,m-1,mat,dp)%1000000007;
  
}

}

This is my solution using 2d ArrayList:

import java.util.*;

public class Solution {

public static int f(int i, int j, ArrayList<ArrayList<Integer>> mat, ArrayList<ArrayList<Integer>> dp){

    if(i == 0 && j == 0)
    return 1;
  if((i < 0) || j < 0 || (mat.get(i).get(j)) == -1)
    return 0;
    if(dp.get(i).get(j) != -1)
    return dp.get(i).get(j);
   
   
    return dp.get(i).set(j , f(i-1,j,mat,dp)%1000000007+f(i,j-1,mat,dp)%1000000007);
   

}

static int mazeObstacles(int n, int m, ArrayList<ArrayList<Integer>> mat) {
    // Write your code here.
    ArrayList<ArrayList<Integer>> dp = new ArrayList<>();
     for (int i = 0; i < n; i++) {
        ArrayList<Integer> row = new ArrayList<>();
        for (int j = 0; j < m; j++) {
            row.add(-1);  // Add 0 to each column in the row
        }
        dp.add(row);  // Add the row to the 2D ArrayList
    }
   return f(n-1,m-1,mat,dp)%1000000007;
  
}

}

I’m trying to initialize a 2d arrayList with -1 initially and based on that am accumulating my DP array.

pls help me where i went wrong when using 2d ArrayList.