A diamond is two pyramids stacked back to back. The top pyramid widens as it goes down, and the bottom pyramid is the same rows in reverse, narrowing back to a point. Together they form a symmetric figure 2n - 1 rows tall and, at its widest middle row, 2n - 1 characters across.

The building block is the pyramid row. For row k, counting from 1 at the tip to n at the base, the line carries n - k leading spaces, then 2k - 1 stars. As k grows, the leading spaces shrink by one and the stars grow by two, keeping every row centered while the star count climbs through the odd numbers 1, 3, 5, and so on.

For n = 4, the top pyramid is *, ***, *****, *******. The bottom half then repeats rows three, two, and one, giving *****, ***, and *. There are no trailing spaces, so each row ends right after its last star.

Key Constraints:

Intuition

Build the diamond in two passes. The first walks k from 1 to n, adding a row of n - k spaces and 2k - 1 stars each time, covering the tip down through the widest middle row.

The second pass walks k back down from n - 1 to 1. The same formula reproduces the upper rows in reverse, the mirror image. Starting at n - 1 leaves the middle row out of the second pass so it appears only once.

Algorithm

Example Walkthrough

Input:

With n = 4, the top pass runs k from 1 to 4. At k = 1 the row is 3 spaces and 1 star, *. At k = 2 it is 2 spaces and 3 stars, ***. At k = 3 it is *****, and at k = 4 it is ******* with no leading space and seven stars. The bottom pass runs k from 3 down to 1, reproducing *****, ***, and *. Joining both halves gives the full diamond:

Code

Writing the two halves separately repeats the same row-building formula twice. A single loop can cover the whole diamond by reasoning about how far each row sits from the middle.

Intuition

Each row's shape is decided by how far it sits from the middle row. The middle row is row n, out of 2n - 1 rows. A row d steps from the middle, in either direction, has d leading spaces and 2 × (n - d) - 1 stars. The middle row has d = 0, so no spaces and the full 2n - 1 stars, and the two tips are d = n - 1 away with one star each.

This collapses the figure into one loop. Walk a row index r from 1 to 2n - 1, compute d as the absolute difference between r and n, then build the row from d and n. Since d is symmetric around the middle, both halves come out of the same formula.

Algorithm

Example Walkthrough

Input:

With n = 4, the middle is row 4, and r runs from 1 to 7. At r = 1, the distance is d = 3, so the row has 3 spaces and 2 × (4 - 3) - 1 = 1 star, giving *. At r = 2, d = 2 produces ***. At r = 3, d = 1 produces *****. At r = 4, d = 0 produces *******. Past the middle the distances climb again: r = 5 has d = 1, r = 6 has d = 2, and r = 7 has d = 3, mirroring the top rows. The single loop builds the whole diamond:

Code