根据下图，最少需要添加多少个单位立方体才能组成一个长方体？ 答案：___

1. Determine the dimensions of the minimum cuboid that can enclose the structure.

The maximum height is $3$ cubes.
The maximum length is $6$ cubes (1 corner column + 2 columns + 3 columns).
The maximum width is $4$ cubes (1 corner column + 3 cubes extending forward).
Dimensions of the minimum cuboid = $6 \times 4 \times 3$.

2. Calculate the total number of unit cubes required for the full cuboid.

Total cubes for the cuboid = $6 \times 4 \times 3 = 72$

3. Find the minimum number of unit cubes to be added.

Number of cubes to add = Total cubes in cuboid $-$ Existing cubes
Number of cubes to add = $72 - 13 = 59$