1299. Replace Elements with Greatest Element on Right Side

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

Constraints:

• `1 <= arr.length <= 104`
• `1 <= arr[i] <= 105`

1299. Replace Elements with Greatest Element on Right Side
``````struct Solution;
use std::i32;

impl Solution {
fn replace_elements(mut arr: Vec<i32>) -> Vec<i32> {
let mut max = -1;
let n = arr.len();
for i in (0..n).rev() {
let t = max;
max = i32::max(arr[i], max);
arr[i] = t;
}
arr
}
}

#[test]
fn test() {
let arr = vec![17, 18, 5, 4, 6, 1];
let res = vec![18, 6, 6, 6, 1, -1];
assert_eq!(Solution::replace_elements(arr), res);
}
``````