Leetcode 62. Unique Paths

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