0162EasyArrayPrefix Sum

Climb Altitude Log

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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 trekking watch splits a walk into legs and stores one signed altitude change per leg in deltas, measured in metres. A positive entry is a climb, a negative entry is a descent, and a zero entry is flat ground.

The watch starts the walk at altitude 0. Build the log the walker actually wants: an array of the same length whose entry i is the altitude reached once leg i has been finished, which is the sum of deltas[0] through deltas[i].

Altitudes may sit below the starting point, so entries in the answer can be negative.

Examples

Example 1

Input: deltas = [12, -5, 40, 0]
Output: [12, 7, 47, 47]

After the first leg the walker is 12 metres up, the descent brings that to 7, the long climb takes it to 47, and the flat leg leaves it there.

Example 2

Input: deltas = [-3, -3, -3]
Output: [-3, -6, -9]

Every leg drops three metres, so the log runs steadily below the starting altitude.

Example 3

Input: deltas = [7]
Output: [7]

A single leg gives a single altitude, which is that leg's own change.

Constraints

1 <= deltas.length <= 1000
-10^6 <= deltas[i] <= 10^6

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 altitude_log(deltas: list[int]) -> list[int]:

Java

public int[] altitudeLog(int[] deltas)

The intended approach

Time O(n)Space O(1) beyond the returned log

The altitude after leg `i` is the altitude after leg `i - 1` plus this leg's change, so nothing has to be re-added. Carry one running altitude through a single left-to-right pass and record it after each leg.

Pythonentry point: altitude_log

def altitude_log(deltas):
    # Time O(n): one pass, one addition per leg.
    # Space O(1) beyond the log that is handed back.
    log = []
    altitude = 0
    for change in deltas:
        altitude += change
        log.append(altitude)
    return log

The obvious version

For every leg, re-add all the changes from the start of the walk up to and including that leg, throwing away the work each time.

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_altitude_log(deltas):
    log = []
    for end in range(len(deltas)):
        total = 0
        for index in range(end + 1):
            total += deltas[index]
        log.append(total)
    return log

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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