## 1331. Rank Transform of an Array

Given an array of integers `arr`, replace each element with its rank.

The rank represents how large the element is. The rank has the following rules:

• Rank is an integer starting from 1.
• The larger the element, the larger the rank. If two elements are equal, their rank must be the same.
• Rank should be as small as possible.

Example 1:

```Input: arr = [40,10,20,30]
Output: [4,1,2,3]
Explanation: 40 is the largest element. 10 is the smallest. 20 is the second smallest. 30 is the third smallest.```

Example 2:

```Input: arr = [100,100,100]
Output: [1,1,1]
Explanation: Same elements share the same rank.
```

Example 3:

```Input: arr = [37,12,28,9,100,56,80,5,12]
Output: [5,3,4,2,8,6,7,1,3]
```

Constraints:

• `0 <= arr.length <= 105`
• `-109 <= arr[i] <= 109`

## Rust Solution

``````struct Solution;
use std::collections::BTreeSet;
use std::collections::HashMap;

impl Solution {
fn array_rank_transform(arr: Vec<i32>) -> Vec<i32> {
let mut bts: BTreeSet<i32> = BTreeSet::new();
let mut hm: HashMap<i32, i32> = HashMap::new();
for &x in &arr {
bts.insert(x);
}
for (i, x) in bts.into_iter().enumerate() {
hm.insert(x, i as i32 + 1);
}
arr.into_iter().map(|x| hm[&x]).collect()
}
}

#[test]
fn test() {
let arr = vec![40, 10, 20, 30];
let res = vec![4, 1, 2, 3];
assert_eq!(Solution::array_rank_transform(arr), res);
let arr = vec![100, 100, 100];
let res = vec![1, 1, 1];
assert_eq!(Solution::array_rank_transform(arr), res);
let arr = vec![37, 12, 28, 9, 100, 56, 80, 5, 12];
let res = vec![5, 3, 4, 2, 8, 6, 7, 1, 3];
assert_eq!(Solution::array_rank_transform(arr), res);
}
``````

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