献丑了。。不知道对不对，dp问题，dp[i][j]表示0-i个袋子装j个球的放法数



	public class BallInPackage {

    public int ballInPackage(int numsOfBags, int numsOfBalls)
     {
         int[][] dp = new int[numsOfBags][numsOfBalls + 1];
         //initialization
         for(int i = 0; i < numsOfBags; i++)
             dp[i][0] = 1;
         for(int i = 1; i <= numsOfBalls; i++)
             dp[0][i] = 1;
         //calculate the dp matrix
         for(int i = 1; i < numsOfBags; i++) {
             for(int j = 1; j <= numsOfBalls; j++) {
                 int nums = 0;
                 for(int k = 0; k <= j; k++)
                     nums += dp[i-1][k];
                 dp[i][j] = nums;
             }
         }
         return dp[numsOfBags-1][numsOfBalls];
     }
     public int ballInPackage_Advanced(int numsOfBags, int numsOfBalls)
     {
         int[][] dp = new int[numsOfBags][numsOfBalls + 1];
         //initialization
         for(int i = 0; i < numsOfBags; i++)
             dp[i][0] = 1;
         dp[0][2] = 1; dp[0][3] = 1;
         //calculate the dp matrix
         for(int i = 1; i < numsOfBags; i++) {
             for(int j = 1; j <= numsOfBalls; j++) {
                 if(j < 2)
                     dp[i][j] += dp[i-1][j];
                 else if(j == 2)
                     dp[i][j] += dp[i-1][j] + dp[i-1][j-2];
                 else if(j >= 3)
                     dp[i][j] += dp[i-1][j] + dp[i-1][j-2] + dp[i-1][j-3];
                 else
                     dp[i][j] = 0;
             }
         }
         return dp[numsOfBags-1][numsOfBalls];
     }
     public static void main(String[] args) {
         BallInPackage b = new BallInPackage();
         System.out.println(b.ballInPackage(12, 20));
         System.out.println(b.ballInPackage_Advanced(12, 20));
     }
 }