0132EasyArrayTwo Pointers

Discard Fault Samples

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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 soil probe writes one reading per minute into a fixed buffer samples. One reading value, fault, is a diagnostic code rather than a measurement, so every occurrence of it has to be squeezed out of the buffer.

Rewrite samples in place so that its first kept slots hold exactly the readings that differ from fault, keeping those readings in the order they were recorded. Slots from index kept onward may hold anything; nobody reads them. You may not allocate a second buffer that grows with the input, so the rewrite has to use O(1) extra space.

Because the grader can only inspect a return value, return the pair [kept, samples[:kept]]: the number of surviving readings followed by the surviving prefix itself. The prefix must be in recorded order, so the answer is unique.

Examples

Example 1

Input: samples = [4, 9, 4, 7, 4], fault = 4
Output: [2, [9, 7]]

Three diagnostic codes drop out; the two real readings slide left and keep their recorded order.

Example 2

Input: samples = [21, 8, 21, 21, 13, 8], fault = 21
Output: [3, [8, 13, 8]]

The buffer starts with a code and holds a run of two more in the middle, so the three survivors compact into the first three slots.

Example 3

Input: samples = [12], fault = 40
Output: [1, [12]]

No reading matches the code, so nothing moves and the whole buffer survives.

Constraints

0 <= samples.length <= 100
0 <= samples[i] <= 50
0 <= fault <= 100

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 discard_fault_samples(samples: list[int], fault: int) -> list:

Java

public int[] discardFaultSamples(int[] samples, int fault)

The intended approach

Time O(n)Space O(1) extra space; the returned prefix is a copy made only for reporting

Keep a write cursor behind a read cursor. The read cursor scans every slot; whenever it sees a reading that is not the diagnostic code, that reading is copied to the write cursor and the cursor advances. The write cursor never overtakes the read cursor, so no surviving reading is clobbered before it is copied, and order is preserved.

Pythonentry point: discard_fault_samples

def discard_fault_samples(samples, fault):
    kept = 0
    for i in range(len(samples)):
        if samples[i] != fault:
            samples[kept] = samples[i]
            kept += 1
    return [kept, samples[:kept]]

The obvious version

Ignore the in-place requirement entirely: filter the buffer into a fresh list and report its length together with the list. Obviously correct, and it ignores the space budget the real solution must respect.

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_discard_fault_samples(samples, fault):
    survivors = []
    for value in samples:
        if value != fault:
            survivors.append(value)
    return [len(survivors), survivors]

How this problem was checked

Reference solution executed against 10 test cases — 10 passed
3 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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