The day the account went under
A cash-flow view replays a month of movements and flags the first day the balance dropped below zero.
- The balance starts at zero and each movement is applied in order.
- Days are numbered from 1.
- Return the first day the running balance is below zero.
- If it never goes under, return 0.
first_day_under(movements: list<int>) → int
Where you start
def first_day_under(movements: list[int]) -> int:
Worked examples
| Call | Result |
|---|---|
first_day_under([100, -30, -90]) | 3 |
first_day_under([100, 50]) | 0 |
first_day_under([-1]) | 1 |
first_day_under([]) | 0 |
Hint
This is a running total with one test each step. There is nothing to look back at — you only need the balance so far.
Reference solution in Python
def first_day_under(movements: list[int]) -> int:
balance = 0
for i, movement in enumerate(movements):
balance += movement
if balance < 0:
return i + 1
return 0