## 41. First Missing Positive

Given an unsorted integer array `nums`, find the smallest missing positive integer.

Follow up: Could you implement an algorithm that runs in `O(n)` time and uses constant extra space.?

Example 1:

```Input: nums = [1,2,0]
Output: 3
```

Example 2:

```Input: nums = [3,4,-1,1]
Output: 2
```

Example 3:

```Input: nums = [7,8,9,11,12]
Output: 1
```

Constraints:

• `0 <= nums.length <= 300`
• `-231 <= nums[i] <= 231 - 1`

## Rust Solution

``````struct Solution;

impl Solution {
fn first_missing_positive(mut nums: Vec<i32>) -> i32 {
let n = nums.len();
for i in 0..n {
loop {
let j = nums[i] - 1;
if j >= 0 && j < n as i32 && nums[j as usize] != nums[i] {
nums.swap(i, j as usize);
} else {
break;
}
}
}
for i in 0..n {
if nums[i] != i as i32 + 1 {
return (i + 1) as i32;
}
}
(n + 1) as i32
}
}

#[test]
fn test() {
let nums = vec![1, 2, 0];
let res = 3;
assert_eq!(Solution::first_missing_positive(nums), res);
let nums = vec![3, 4, -1, 1];
let res = 2;
assert_eq!(Solution::first_missing_positive(nums), res);
let nums = vec![7, 8, 9, 11, 12];
let res = 1;
assert_eq!(Solution::first_missing_positive(nums), res);
}
``````

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