unique paths

unique paths
There is a robot on an m x n grid. The robot is initially located at the top-left corner (i.e., grid[0][0]). 
The robot tries to move to the bottom-right corner (i.e., grid[m - 1][n - 1]). The robot can only move either down or right at any point in time.
Given the two integers m and n, return the number of possible unique paths that the robot can take to reach the bottom-right corner.
The test cases are generated so that the answer will be less than or equal to 2 * 109.

Example 1
Input: m = 3, n = 7
Output: 28

Example 2:
Input: m = 3, n = 2
Output: 3
Explanation: From the top-left corner, there are a total of 3 ways to reach the bottom-right corner:
1. Right -> Down -> Down
2. Down -> Down -> Right
3. Down -> Right -> Down

class Solution {
    public int uniquePaths(int m, int n) {
        int[][]dp=new int[m+1][n+1];
        for(int[]arr:dp){
            Arrays.fill(arr,-1);
        }
        return helper(m,n,dp);
    }
    private int helper (int m,int n,int[][]dp){
        if(m==1&&n==1){
            return 1;
        }
        if(dp[m][n]!=-1){
            return dp[m][n];
        }
        int count=0;
        if(m>=1){
            count=count+helper(m-1,n,dp);
        }
        if(n>=1){
            count=count+helper(m,n-1,dp);
        }
        return dp[m][n]=count;

    }
}