0308EasyTwo PointersString

Alternating Block Flip

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Trains the technique from

This is an original problem, written from a brief that listed the technique, the difficulty, the topics, the function shape and the input bounds — none of that problem's wording, examples, hints or editorials. The link is there so you can map your practice onto the standard set.

Same function shape, different story and different numbers.

A ribbon printer receives a strip of lowercase letters, strip, and a block size, block.

Reading from the left, cut the strip into consecutive blocks of block letters each. The last block is whatever is left over, so it may hold fewer than block letters. Number the blocks 1, 2, 3, ... from the left.

The printer flips every odd-numbered block, that is blocks 1, 3, 5, ..., reversing the order of the letters inside that block only. Even-numbered blocks come out untouched. A short final block is flipped just the same if its number is odd.

Return the strip the printer produces.

Examples

Example 1

Input: strip = "photograph", block = 3
Output: "ohptogparh"

The blocks are "pho", "tog", "rap" and "h". Flipping blocks 1 and 3 gives "ohp", "tog", "par" and "h", which join into "ohptogparh".

Example 2

Input: strip = "carousel", block = 3
Output: "racousle"

The blocks are "car", "ous" and "el". Block 3 holds only two letters and is still flipped, so the pieces become "rac", "ous" and "le".

Example 3

Input: strip = "windmill", block = 5
Output: "mdniwill"

The blocks are "windm" and "ill". Block 1 flips to "mdniw" and block 2 is left alone.

Example 4

Input: strip = "kite", block = 9
Output: "etik"

The block size is larger than the strip, so the whole strip is block 1 and the entire thing is flipped.

Example 5

Input: strip = "lantern", block = 1
Output: "lantern"

Every block holds a single letter, and flipping one letter changes nothing.

Constraints

1 <= strip.length <= 10^4
strip consists of only lowercase English letters.
1 <= block <= 10^4

The signature

The editor is preloaded with this. It matches the parent problem's shape, so a solution that works here transfers to a judge unchanged.

Python

def flip_blocks(strip: str, block: int) -> str:

Java

public String flipBlocks(String strip, int block)

The intended approach

Time O(n)Space O(n) for the character buffer that becomes the returned strip

Copy the strip into a mutable list of characters and step the start index forward by `2 * block`, so each stop lands on a block that needs flipping. At each stop, walk a left and a right pointer toward each other and swap, clamping the right pointer to the final character so a short trailing block is handled by the same loop.

Pythonentry point: flip_blocks

def flip_blocks(strip, block):
    # O(n) time: every character is touched at most once by a swap.
    # O(n) space, which is the buffer holding the strip we return.
    chars = list(strip)
    n = len(chars)
    for start in range(0, n, 2 * block):
        i = start
        j = min(start + block, n) - 1
        while i < j:
            chars[i], chars[j] = chars[j], chars[i]
            i += 1
            j -= 1
    return ''.join(chars)

The obvious version

Slice the strip into blocks one after another, reverse the odd-numbered slices with a slice step, and concatenate the pieces.

This is not filler. It was written independently of the fast solution and the two were run against thousands of random inputs to confirm they agree — which is how the answer above is known to be right. Read it when the intended approach feels like a rabbit pulled out of a hat.

Python

def bf_flip_blocks(strip, block):
    pieces = []
    idx = 0
    number = 1
    while idx < len(strip):
        piece = strip[idx:idx + block]
        if number % 2 == 1:
            piece = piece[::-1]
        pieces.append(piece)
        idx += block
        number += 1
    return ''.join(pieces)

How this problem was checked

Reference solution executed against 12 test cases — 12 passed
5 worked examples confirmed by running the code
0 eight-word sequences shared with the parent problem's text
Agrees with an independent brute-force implementation on random inputs
Test set proven to catch deliberately broken solutions
Reviewed by a separate quality pass
Function arity matches the parent problem's

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