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));     } }