A robot is located at the top-left corner of a m x n grid (marked 'Start' in the diagram below).

The robot can only move either down or right at any point in time. The robot is trying to reach the bottom-right corner of the grid (marked 'Finish' in the diagram below).

How many possible unique paths are there?

public class Solution {
    public int uniquePaths(int m, int n) {
        if (m < 1 || n < 1)
            return 0;
        int[][] res = new int[m][n];
        for(int i = 0; i < m; i++){
            for(int j = 0; j < n; j++){
                res[i][j] = 0;
            }
        }
        for(int i = 0; i < m; i++){
            res[i][0] = 1;
        }
        for(int i = 0; i < n; i++){
            res[0][i] = 1;
        }
        for(int i = 1; i < m; i++){
            for(int j = 1; j < n; j++)
                res[i][j] = res[i][j-1] + res[i-1][j];
        }
        return res[m-1][n-1];
    }
}