LeetCode 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 < 0 || n < 0 ){
            return 0;
        }
        if(m == 0 || n == 0){
            return 1;
        }
        int[][] matrix = new int[m][n];
        for(int i = 0 ; i < n; i++){
             matrix[0][i] = 1; 
        }
        for(int j = 0; j < m; j++){
            matrix[j][0] = 1;
        }

        for(int i =1; i < m ; i++){
            for(int j = 1; j < n; j++){
                matrix[i][j] = matrix[i-1][j] + matrix[i][j-1];
            }
        }
        return matrix[m-1][n-1];
    }
}

要到达一个点，就是要从左边或者上边到达，所以，用动态规划存到达每个点的情况。