Print a Hollow Hourglass Star Pattern

Easy15 min

Topics

BasicsPatternsLoops

Companies

TCSInfosysWipro

Given n, return the 2n − 1 rows of a hollow hourglass: a solid top row of 2n − 1 stars, rows that narrow to a single-star waist, then rows that widen back out to a solid bottom row. Between the lids only the outline is drawn. The hourglass is a reverse pyramid stacked on a pyramid, sharing the middle row. A single number k = |n − 1 − r| + 1, built with the absolute value, rises and falls in a V shape and drives both halves at once, so there is no need for two separate loops.

Test Case 1:

Input:n = 3

Output:["*****", " * *", " *", " * *", "*****"]

Explanation:Solid top and bottom rows, a single-star waist in the middle, and only the outline in between.

Test Case 2:

Input:n = 1

Output:["*"]

Explanation:The top, waist and bottom are all the same single row.

Constraints

◆1 ≤ n ≤ 20

Try the Dry Run

Mode

Approach

Approach & Solutions

Nested Loops — Edge Check per CellGood

There are 2n − 1 rows. For row r let k = |n − 1 − r| + 1: it falls from n to 1 at the middle row and climbs back to n. Each row has n − k leading spaces and 2k − 1 cells; a cell is a star if it is the first or last of the row, or if the row is the top or bottom (k = n).

TimeO(n²)

SpaceO(n²) for the output

Java

Try it out

class Solution {
    public String[] printHollowHourglass(int n) {
        String[] result = new String[2 * n - 1];
        for (int r = 0; r < 2 * n - 1; r++) {
            int k = Math.abs(n - 1 - r) + 1;
            StringBuilder row = new StringBuilder();
            for (int s = 0; s < n - k; s++) {
                row.append(' ');
            }
            for (int j = 1; j <= 2 * k - 1; j++) {
                boolean edge = j == 1 || j == 2 * k - 1 || k == n;
                row.append(edge ? '*' : ' ');
            }
            result[r] = row.toString();
        }
        return result;
    }
}

Pick the Row Body by Its WidthOptimal

Classify each row by k: k = n gives a solid body of 2n − 1 stars, k = 1 gives a single star, anything else is "*", 2k − 3 spaces and "*". Prefix n − k spaces. C fills the row with spaces (or stars for the solid rows) using memset and then sets the two edge characters.