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.
firstDayUnder(movements: list<int>) → int
Go needs a compiler and Drill does not host one yet, so this page is the reference rather than an exercise: the problem, worked examples, and the solution in full. To type it out, the same problem runs in Python.
Where you start
func firstDayUnder(movements []int) int {
}Worked examples
| Call | Result |
|---|---|
firstDayUnder([]int{100, -30, -90}) | 3 |
firstDayUnder([]int{100, 50}) | 0 |
firstDayUnder([]int{-1}) | 1 |
firstDayUnder([]int{}) | 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 Go
func firstDayUnder(movements []int) int {
balance := 0
for i, movement := range movements {
balance += movement
if balance < 0 {
return i + 1
}
}
return 0
}