## 414. Third Maximum Number

Given integer array `nums`, return the third maximum number in this array. If the third maximum does not exist, return the maximum number.

Example 1:

```Input: nums = [3,2,1]
Output: 1
Explanation: The third maximum is 1.
```

Example 2:

```Input: nums = [1,2]
Output: 2
Explanation: The third maximum does not exist, so the maximum (2) is returned instead.
```

Example 3:

```Input: nums = [2,2,3,1]
Output: 1
Explanation: Note that the third maximum here means the third maximum distinct number.
Both numbers with value 2 are both considered as second maximum.
```

Constraints:

• `1 <= nums.length <= 104`
• `-231 <= nums[i] <= 231 - 1`

Follow up: Can you find an `O(n)` solution?

## Rust Solution

``````struct Solution;
use std::cmp::Ordering::*;

impl Solution {
fn third_max(nums: Vec<i32>) -> i32 {
let mut max1 = nums;
let mut m2: Option<i32> = None;
let mut m3: Option<i32> = None;
for x in nums {
match x.cmp(&max1) {
Greater => {
m3 = m2;
m2 = Some(max1);
max1 = x;
}
Less => {
if let Some(max2) = m2 {
match x.cmp(&max2) {
Greater => {
m3 = m2;
m2 = Some(x);
}
Less => {
if let Some(max3) = m3 {
if x > max3 {
m3 = Some(x);
}
} else {
m3 = Some(x);
}
}
Equal => {}
}
} else {
m2 = Some(x);
}
}
Equal => {}
}
}
if let Some(max3) = m3 {
max3
} else {
max1
}
}
}

#[test]
fn test() {
let nums = vec![3, 2, 1];
assert_eq!(Solution::third_max(nums), 1);
let nums = vec![1, 2];
assert_eq!(Solution::third_max(nums), 2);
let nums = vec![2, 2, 3, 1];
assert_eq!(Solution::third_max(nums), 1);
let nums = vec![1, 2, 2, 5, 3, 5];
assert_eq!(Solution::third_max(nums), 2);
}
``````

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