# Leetcode - Replace Elements with Greatest Element on Right Side Solution

Given an array `arr`

, replace every element in that array with the greatest element among the elements to its right, and replace the last element with `-1`

.

After doing so, return the array.

**Example 1:**

```
Input: arr = [17,18,5,4,6,1]
Output: [18,6,6,6,1,-1]
Explanation:
- index 0 --> the greatest element to the right of index 0 is index 1 (18).
- index 1 --> the greatest element to the right of index 1 is index 4 (6).
- index 2 --> the greatest element to the right of index 2 is index 4 (6).
- index 3 --> the greatest element to the right of index 3 is index 4 (6).
- index 4 --> the greatest element to the right of index 4 is index 5 (1).
- index 5 --> there are no elements to the right of index 5, so we put -1.
```

**Example 2:**

```
Input: arr = [400]
Output: [-1]
Explanation: There are no elements to the right of index 0.
```

**Constraints:**

`1 <= arr.length <= 10`

^{4}`1 <= arr[i] <= 10`

^{5}

## Solution in Python

```
from collections import deque
class Solution:
def replaceElements(self, arr: List[int]) -> List[int]:
max_val = 0
max_arr = deque([-1])
for i in arr[:0:-1]:
max_val = max(max_val,i)
max_arr.appendleft(max_val)
return list(max_arr)
```