# 1488. Avoid Flood in The City

#### Medium

***

Your country has an infinite number of lakes. Initially, all the lakes are empty, but when it rains over the `nth` lake, the `nth` lake becomes full of water. If it rains over a lake which is **full of water**, there will be a **flood**. Your goal is to avoid the flood in any lake.

Given an integer array `rains` where:

* `rains[i] > 0` means there will be rains over the `rains[i]` lake.
* `rains[i] == 0` means there are no rains this day and you can choose **one lake** this day and **dry it**.

Return *an array `ans`* where:

* `ans.length == rains.length`
* `ans[i] == -1` if `rains[i] > 0`.
* `ans[i]` is the lake you choose to dry in the `ith` day if `rains[i] == 0`.

If there are multiple valid answers return **any** of them. If it is impossible to avoid flood return **an empty array**.

Notice that if you chose to dry a full lake, it becomes empty, but if you chose to dry an empty lake, nothing changes. (see example 4)

 

**Example 1:**

```
Input: rains = [1,2,3,4]
Output: [-1,-1,-1,-1]
Explanation: After the first day full lakes are [1]
After the second day full lakes are [1,2]
After the third day full lakes are [1,2,3]
After the fourth day full lakes are [1,2,3,4]
There's no day to dry any lake and there is no flood in any lake.
```

**Example 2:**

```
Input: rains = [1,2,0,0,2,1]
Output: [-1,-1,2,1,-1,-1]
Explanation: After the first day full lakes are [1]
After the second day full lakes are [1,2]
After the third day, we dry lake 2. Full lakes are [1]
After the fourth day, we dry lake 1. There is no full lakes.
After the fifth day, full lakes are [2].
After the sixth day, full lakes are [1,2].
It is easy that this scenario is flood-free. [-1,-1,1,2,-1,-1] is another acceptable scenario.
```

**Example 3:**

```
Input: rains = [1,2,0,1,2]
Output: []
Explanation: After the second day, full lakes are  [1,2]. We have to dry one lake in the third day.
After that, it will rain over lakes [1,2]. It's easy to prove that no matter which lake you choose to dry in the 3rd day, the other one will flood.
```

 

**Constraints:**

* `1 <= rains.length <= 105`
* `0 <= rains[i] <= 109`

```python
from collections import defaultdict
class Solution:
    def avoidFlood(self, rains: List[int]) -> List[int]:
        lakes = collections.defaultdict(bool)
        lastrain = collections.defaultdict(int)
        res = []
        dry = []
        for k,v in enumerate(rains):
            if v > 0:
                if not lakes[v]:
                    lakes[v] = True
                    res.append(-1)
                    lastrain[v] = k
                else:
                    # lakes[v] == True
                    if dry == []:
                        return []
                    else:
                        # check if there is a dry day we can use
                        i = 0
                        found = False
                        while i < len(dry) and not found:
                            if dry[i] > lastrain[v]:
                                found = True
                                dry_day = dry[i]
                            else:
                                i += 1
                        if found:
                            res[dry_day] = v
                            lastrain[v] = k
                            dry.pop(i)
                            res.append(-1)
                        else:
                            return []
            elif v == 0:
                res.append(1)
                dry.append(k)
        return res
```