S3Q2 · Zigzag Number Grid

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Type: Full I/O - pattern printing

Core idea: odd rows go left to right, even rows go right to left. i % 2 decides the direction.

n = int(input())
row = list(range(1, n + 1))
for i in range(1, n + 1):
    print(*(row if i % 2 == 1 else row[::-1]))

Key rules: - Rows are 1-indexed: row 1, 2, 3, … - i % 2 == 1 (odd) → 1 2 3 ... n - i % 2 == 0 (even) → n n-1 ... 1 - print(*iterable) prints space-separated values automatically

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Problem Statement

Problem (I/O type)

Read an integer n. Print an n × n grid where odd rows go left to right and even rows go right to left.

Example:

Input

4

Output

1 2 3 4
4 3 2 1
1 2 3 4
4 3 2 1

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Understanding the pattern

Every row contains the same numbers 1 through n - the only difference is direction:

Row 1 (odd)  →  1  2  3  4      range(1, n+1)
Row 2 (even) ←  4  3  2  1      range(n, 0, -1)  or  row[::-1]
Row 3 (odd)  →  1  2  3  4
Row 4 (even) ←  4  3  2  1

Key insight: i % 2 == 1 → left to right | i % 2 == 0 → right to left

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Tracing n = 4

i	i % 2	Direction	Output
1	1 (odd)	→ left to right	1 2 3 4
2	0 (even)	← right to left	4 3 2 1
3	1 (odd)	→ left to right	1 2 3 4
4	0 (even)	← right to left	4 3 2 1

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Solution approaches

Pythonic (ternary)Explanatory (if-else)Using range with stepUsing lambda

n = int(input())
row = list(range(1, n + 1))
for i in range(1, n + 1):
    print(*(row if i % 2 == 1 else row[::-1]))

n = int(input())
row = list(range(1, n + 1))
for i in range(1, n + 1):
    if i % 2 == 1:
        print(*row)
    else:
        print(*row[::-1])

n = int(input())
for i in range(1, n + 1):
    if i % 2 == 1:
        print(*range(1, n + 1))
    else:
        print(*range(n, 0, -1))

Uses range directly - avoids building a list. range(n, 0, -1) counts down from n to 1.

n = int(input())
row = list(range(1, n + 1))
make_row = lambda i: row if i % 2 == 1 else row[::-1]
for i in range(1, n + 1):
    print(*make_row(i))

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Key takeaways

01

i % 2 controls direction

Odd row → print left to right (range(1, n+1)). Even row → print right to left (range(n, 0, -1) or row[::-1]). Build the row once, reuse it every iteration.

02

row[::-1] reverses in one step

Slicing with step -1 creates a reversed copy without modifying the original. row stays [1, 2, 3, 4] for the next odd row.

03

print(*iterable) adds spaces automatically

The * unpacks the list and print() joins elements with spaces by default. No manual " ".join() or loop needed.