10. (12 points) Please fill in each arrow with an appropriate number, so that the number in the arrow indicates how many different numbers are in the direction the arrow points. Then the four-digit number $\overline{\mathrm{ABCD}}$ is. (The figure below is an example of a $3 \times 3$ grid).





Translate the text above into English, please keep the original text's line breaks and format, and output the translation result directly.
2112
【Solution】Solution: As shown in the figure,
From the first of the second row, the third of the second row, the second of the third row, the arrow only points to one arrow, the number at this position can only be 1, as shown by the red number in the figure,
The first arrow in the third row points to two different arrows, so it can only be 2, so the first position of the fourth row must be 3,
If the second position of the fourth row is 1, then the third of this row must be 3, but it does not match the fourth number of this row, so, the number on the second arrow of the fourth row can only be 2, the third number of this row can only be 1, which means all the numbers in the third column are 1,
The second and fourth of the second row are also 2, so the second number of the first row is also 2,
The fourth of the first row can only be 3, the fourth of the third row must be 2,
That is: The numbers at positions $A, B, C, D$ are $2,1,1,2$,
Therefore, the answer is 2112.