62. Unique Paths

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 1;
      //  return uniquePaths(m-1,n)+uniquePaths(m,n-1);
      int[][] a = new int[m][n];
      for(int i=0;i<m;i++)
         a[i][0] = 1;
      for(int j=0;j<n;j++)
         a[0][j] = 1;
      for(int i=1;i<m;i++)
        for(int j=1;j<n;j++)
          a[i][j] = a[i-1][j] + a[i][j-1];
      return a[m-1][n-1];
    }
}