forecast goes day by day: forecast[i] > 0 means that on day i rain fills the pond with that number, and forecast[i] == 0 means a dry day on which you may drain any one pond. If rain falls on a pond that is already full, it overflows.
Build a plan plan of the same length: on rainy days plan[i] = -1, and on dry days it is the number of the pond being drained. To make the answer unique, build the plan this way. Go through the days in order. When rain comes back to a pond that was filled on day p, give that pond the earliest dry day after p that has not been taken yet; if there is no such day, an overflow cannot be avoided, so return an empty array. Dry days that end up unused go to pond 1 (draining an empty pond is allowed and changes nothing).
Try to avoid a quadratic scan over the dry days.forecast=[3, 0, 5, 0, 3, 5] → [-1, 3, -1, 5, -1, -1]
Rain over pond 3 returns on day 4, so it is drained on the first free dry day after day 0, which is day 1. Pond 5 (rain on days 2 and 5) gets day 3. The answer is [-1, 3, -1, 5, -1, -1].
forecast=[2, 0, 0, 7] → [-1, 1, 1, -1]
No pond gets rain twice, so the dry days are not needed and, by the rule, pond 1 is drained on them.
forecast=[4, 6, 0, 4, 6] → []
The only dry day goes to pond 4, and pond 6 overflows on day 4. The answer is an empty array.
forecast=[3, 0, 5, 0, 3, 5]
[-1, 3, -1, 5, -1, -1]
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