0303EasyArray

Paired Halves Interleave

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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 flow logger keeps one batch in a single flat list readings of length 2 * half. The first half entries are the intake figures for pairs 0, 1, ..., half - 1 in that order, and the last half entries are the outflow figures for the same pairs in the same order, so readings[half + i] is the outflow that belongs with the intake readings[i].

Rebuild the batch pair by pair: the intake of pair 0, then the outflow of pair 0, then the intake of pair 1, then the outflow of pair 1, and so on to the last pair.

Return the rebuilt list, which holds the same 2 * half figures in that paired order.

Examples

Example 1

Input: readings = [3,7,2,9,4,1], half = 3
Output: [3,9,7,4,2,1]

The intake figures are 3, 7 and 2 and the outflow figures are 9, 4 and 1, so pair 0 reads 3 then 9, pair 1 reads 7 then 4, and pair 2 reads 2 then 1.

Example 2

Input: readings = [5,4,3,2,1,6], half = 3
Output: [5,2,4,1,3,6]

Intake 5 belongs with outflow 2, intake 4 with outflow 1, and intake 3 with outflow 6.

Constraints

1 <= half <= 500
readings.length == 2 * half
1 <= readings[i] <= 10^3

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 interleave_halves(readings: list[int], half: int) -> list[int]:

Java

public int[] interleaveHalves(int[] readings, int half)

The intended approach

Time O(n)Space O(n) for the returned list

Run a single index over the pair numbers. For each pair number, append the intake entry at that index and then the outflow entry sitting half a list further along, which lands the two figures of every pair side by side in the result.

Pythonentry point: interleave_halves

def interleave_halves(readings, half):
    # Walk one index across both halves at once, emitting the intake entry and then
    # the outflow entry that shares its pair number.
    # O(n) time, O(n) space for the returned list of 2 * half entries.
    out = []
    for i in range(half):
        out.append(readings[i])
        out.append(readings[half + i])
    return out

The obvious version

Allocate the result up front and fill it by output position, reading from the intake half on even positions and the outflow half on odd ones.

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_interleave_halves(readings, half):
    total = 2 * half
    out = [0] * total
    for pos in range(total):
        if pos % 2 == 0:
            out[pos] = readings[pos // 2]
        else:
            out[pos] = readings[half + pos // 2]
    return out

How this problem was checked

Reference solution executed against 7 test cases — 7 passed
2 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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