Carrying Forward

A medium Python interview practice problem on DataDriven. Write the Python and run it against test cases, with instant feedback.

Domain
Python
Difficulty
medium
Seniority
L3

Problem

Given the daily deposits to an account in the order they landed, return the balance standing after each one, for a running-balance chart on a dashboard.

The total grows with every row.

Example 1

Input:

deposits = [1,2,3,4]

Output:

[1,3,6,10]

Example 2

Input:

deposits = []

Output:

[]

Worked solution and explanation

What this really is

This is a prefix sum wearing a ledger costume. The skill being probed: can you carry state across a single pass instead of recomputing it? Anyone can produce the right numbers. The trap is reaching for sum(deposits[:i+1]) at each position, which re-adds the same prefix over and over and turns an O(n) walk into O(n^2) that quietly dies on a 10k-element ledger. Get the accumulator right and the empty list falls out for free, no special case.

Break down the requirements

Step 1: Maintain a running total in one pass

Initialize total = 0, loop over deposits, update total += amount, append total to the output list. Single pass, no nested loops. The naive O(n^2) is sum(deposits[:i+1]) for each i, which recomputes the same prefixes repeatedly and is a red flag at any level above junior.

Step 2: Preallocate or append; both are O(n)

Python list.append has amortized O(1) cost, so the append-in-loop version hits O(n) total. Preallocating [0] * len(deposits) and assigning by index is the same asymptotic cost but uses more code. Append is cleaner and equally fast.

Step 3: Return [] for empty input with no branch

Looping over [] produces no iterations, the output list stays [], the return value is []. No 'if not deposits: return []' branch needed. Writing it is a tell that you do not trust the loop.

The solution

Accumulate in one pass
def running_total(deposits):
    balances = []
    total = 0
    for amount in deposits:
        total += amount
        balances.append(total)
    return balances

Common follow-up questions